diff --git a/egs/librispeech/ASR/local/download_lm.py b/egs/librispeech/ASR/local/download_lm.py
index 47251a5a0..0bdc2935b 100755
--- a/egs/librispeech/ASR/local/download_lm.py
+++ b/egs/librispeech/ASR/local/download_lm.py
@@ -29,10 +29,12 @@ def download_lm():
         filename = target_dir / f
         if filename.is_file() is False:
             urlretrieve_progress(
-                f"{url}/{f}", filename=filename, desc=f"Downloading {filename}",
+                f"{url}/{f}",
+                filename=filename,
+                desc=f"Downloading {filename}",
             )
         else:
-            print(f'{filename} already exists - skipping')
+            print(f"{filename} already exists - skipping")
 
         if ".gz" in str(filename):
             unzip_file = Path(os.path.splitext(filename)[0])
@@ -41,7 +43,7 @@ def download_lm():
                     with open(unzip_file, "wb") as f_out:
                         shutil.copyfileobj(f_in, f_out)
             else:
-                print(f'{unzip_file} already exist - skipping')
+                print(f"{unzip_file} already exist - skipping")
 
 
 if __name__ == "__main__":
diff --git a/egs/librispeech/ASR/local/prepare_lang.py b/egs/librispeech/ASR/local/prepare_lang.py
new file mode 100755
index 000000000..a568cc8a9
--- /dev/null
+++ b/egs/librispeech/ASR/local/prepare_lang.py
@@ -0,0 +1,376 @@
+#!/usr/bin/env python3
+
+# Copyright (c)  2021  Xiaomi Corporation (authors: Fangjun Kuang)
+
+"""
+This script takes as input a lexicon file "data/lang/lexicon.txt"
+consisting of words and phones and does the following:
+
+1. Add disambiguation symbols to the lexicon and generate lexicon_disambig.txt
+
+2. Generate phones.txt, the phones table mapping a phone to a unique integer.
+
+3. Generate words.txt, the words table mapping a word to a unique integer.
+
+4. Generate L.pt, in k2 format. It can be loaded by
+
+        d = torch.load("L.pt")
+        lexicon = k2.Fsa.from_dict(d)
+
+5. Generate L_disambig.pt, in k2 format.
+
+6. Generate lexicon_disambig.txt
+"""
+import math
+import re
+import sys
+from collections import defaultdict
+from pathlib import Path
+from typing import Dict, List, Tuple
+
+import k2
+import torch
+
+Lexicon = List[Tuple[str, List[str]]]
+
+
+def read_lexicon(filename: str) -> Lexicon:
+    """Read a lexicon.txt in `filename`.
+
+    Each line in the lexicon contains "word p1 p2 p3 ...".
+    That is, the first field is a word and the remaining
+    fields are phones. Fields are separated by space(s).
+
+    We assume that the input lexicon does not contain words:
+    <eps>, <s>, </s>, !SIL, <SPOKEN_NOISE>, <UNK>.
+
+    Args:
+      filename:
+        Path to the lexicon.txt
+
+    Returns:
+      A list of tuples., e.g., [('w', ['p1', 'p2']), ('w1', ['p3, 'p4'])]
+    """
+    #  ans = ["!SIL", ["SIL"]]
+    #  ans.append(["<SPOKEN_NOISE>", ["SPN"]])
+    #  ans.append(["<UNK>", ["SPN"]])
+
+    ans = []
+
+    with open(filename, "r", encoding="latin-1") as f:
+        whitespace = re.compile("[ \t]+")
+        for line in f:
+            a = whitespace.split(line.strip(" \t\r\n"))
+            if len(a) == 0:
+                continue
+
+            if len(a) < 2:
+                print(f"Found bad line {line} in lexicon file {filename}")
+                print("Every line is expected to contain at least 2 fields")
+                sys.exit(1)
+            word = a[0]
+            if word == "<eps>":
+                print(f"Found bad line {line} in lexicon file {filename}")
+                print("<eps> should not be a valid word")
+                sys.exit(1)
+
+            prons = a[1:]
+            ans.append((word, prons))
+
+    return ans
+
+
+def write_lexicon(filename: str, lexicon: Lexicon) -> None:
+    """Write a lexicon to a file.
+
+    Args:
+      filename:
+        Path to the lexicon file to be generated.
+      lexicon:
+        It can be the return value of :func:`read_lexicon`.
+    """
+    with open(filename, "w") as f:
+        for word, prons in lexicon:
+            f.write(f"{word} {' '.join(prons)}\n")
+
+
+def write_mapping(filename: str, sym2id: Dict[str, int]) -> None:
+    """Write a symbol to ID mapping to a file.
+
+    Args:
+      filename:
+        Filename to save the mapping.
+      sym2id:
+        A dict mapping symbols to IDs.
+    Returns:
+      Return None.
+    """
+    with open(filename, "w") as f:
+        for sym, i in sym2id.items():
+            f.write(f"{sym} {i}\n")
+
+
+def get_phones(lexicon: Lexicon) -> List[str]:
+    """Get phones from a lexicon.
+
+    Args:
+      lexicon:
+        It is the return value of :func:`read_lexicon`.
+    Returns:
+      Return a list of unique phones.
+    """
+    ans = set()
+    for _, prons in lexicon:
+        ans.update(prons)
+    sorted_ans = sorted(list(ans))
+    return sorted_ans
+
+
+def get_words(lexicon: List[Tuple[str, List[str]]]) -> List[str]:
+    """Get words from a lexicon.
+
+    Args:
+      lexicon:
+        It is the return value of :func:`read_lexicon`.
+    Returns:
+      Return a list of unique words.
+    """
+    ans = set()
+    for word, _ in lexicon:
+        ans.add(word)
+    sorted_ans = sorted(list(ans))
+    return sorted_ans
+
+
+def add_disambig_symbols(lexicon: Lexicon) -> Tuple[Lexicon, int]:
+    """It adds pseudo-phone disambiguation symbols #1, #2 and so on
+    at the ends of phones to ensure that all pronunciations are different,
+    and that none is a prefix of another.
+
+    See also add_lex_disambig.pl from kaldi.
+
+    Args:
+      lexicon:
+        It is returned by :func:`read_lexicon`.
+    Returns:
+      Return a tuple with two elements:
+
+        - The output lexicon with disambiguation symbols
+        - The ID of the max disambiguation symbols
+    """
+
+    # (1) Work out the count of each phone-sequence in the
+    # lexicon.
+    count = defaultdict(int)
+    for _, prons in lexicon:
+        count[" ".join(prons)] += 1
+
+    # (2) For each left sub-sequence of each phone-sequence, note down
+    # that it exists (for identifying prefixes of longer strings).
+    issubseq = defaultdict(int)
+    for _, prons in lexicon:
+        prons = prons.copy()
+        prons.pop()
+        while prons:
+            issubseq[" ".join(prons)] = 1
+            prons.pop()
+
+    # (3) For each entry in the lexicon:
+    # if the phone sequence is unique and is not a
+    # prefix of another word, no disambig symbol.
+    # Else output #1, or #2, #3, ... if the same phone-seq
+    # has already been assigned a disambig symbol.
+    ans = []
+
+    # We start with #1 since #0 has its own purpose
+    first_allowed_disambig = 1
+    max_disambig = first_allowed_disambig - 1
+    last_used_disambig_symbol_of = defaultdict(int)
+
+    for word, prons in lexicon:
+        phnseq = " ".join(prons)
+        assert phnseq != ""
+        if issubseq[phnseq] == 0 and count[phnseq] == 1:
+            ans.append((word, prons))
+            continue
+
+        cur_disambig = last_used_disambig_symbol_of[phnseq]
+        if cur_disambig == 0:
+            cur_disambig = first_allowed_disambig
+        else:
+            cur_disambig += 1
+
+        if cur_disambig > max_disambig:
+            max_disambig = cur_disambig
+        last_used_disambig_symbol_of[phnseq] = cur_disambig
+        phnseq += f" #{cur_disambig}"
+        ans.append((word, phnseq.split()))
+    return ans, max_disambig
+
+
+def generate_id_map(symbols: List[str]) -> Dict[str, int]:
+    """Generate ID maps, i.e., map a symbol to a unique ID.
+
+    Args:
+      symbols:
+        A list of unique symbols.
+    Returns:
+      A dict containing the mapping between symbols and IDs.
+    """
+    return {sym: i for i, sym in enumerate(symbols)}
+
+
+def lexicon_to_fst(
+    lexicon: Lexicon,
+    phone2id: Dict[str, int],
+    word2id: Dict[str, int],
+    sil_phone: str = "SIL",
+    sil_prob: float = 0.5,
+) -> k2.Fsa:
+    """Convert a lexicon to an FST (in k2 format) with optional silence at
+    the beginning and end of the word.
+
+    Args:
+      lexicon:
+        The input lexicon. See also :func:`read_lexicon`
+      phone2id:
+        A dict mapping phones to IDs.
+      word2id:
+        A dict mapping words to IDs.
+      sil_phone:
+        The silence phone.
+      sil_prob:
+        The probability for adding a silence at the beginning and end
+        of the word.
+    Returns:
+      Return an instance of `k2.Fsa` representing the given lexicon.
+    """
+    assert sil_prob > 0.0 and sil_prob < 1.0
+    # CAUTION: we use score, i.e, negative cost.
+    sil_score = math.log(sil_prob)
+    no_sil_score = math.log(1.0 - sil_prob)
+
+    start_state = 0
+    loop_state = 1  # words enter and leave from here
+    sil_state = 2  # words terminate here when followed by silence; this state
+    # has a silence transition to loop_state.
+    next_state = 3  # the next un-allocated state, will be incremented as we go.
+    arcs = []
+
+    assert phone2id["<eps>"] == 0
+    assert word2id["<eps>"] == 0
+
+    eps = 0
+
+    sil_phone = phone2id[sil_phone]
+
+    arcs.append([start_state, loop_state, eps, eps, no_sil_score])
+    arcs.append([start_state, sil_state, eps, eps, sil_score])
+    arcs.append([sil_state, loop_state, sil_phone, eps, 0])
+
+    for word, prons in lexicon:
+        assert len(prons) > 0, f"{word} has no pronunciations"
+        cur_state = loop_state
+
+        word = word2id[word]
+        prons = [phone2id[i] for i in prons]
+
+        for i in range(len(prons) - 1):
+            if i == 0:
+                arcs.append([cur_state, next_state, prons[i], word, 0])
+            else:
+                arcs.append([cur_state, next_state, prons[i], eps, 0])
+
+            cur_state = next_state
+            next_state += 1
+
+        # now for the last phone of this word
+        # It has two out-going arcs, one to the loop state,
+        # the other one to the sil_state.
+        i = len(prons) - 1
+        w = word if i == 0 else eps
+        arcs.append([cur_state, loop_state, prons[i], w, no_sil_score])
+        arcs.append([cur_state, sil_state, prons[i], w, sil_score])
+
+    final_state = next_state
+    arcs.append([loop_state, final_state, -1, -1, 0])
+    arcs.append([final_state])
+
+    arcs = sorted(arcs, key=lambda arc: arc[0])
+    arcs = [[str(i) for i in arc] for arc in arcs]
+    arcs = [" ".join(arc) for arc in arcs]
+    arcs = "\n".join(arcs)
+
+    fsa = k2.Fsa.from_str(arcs, acceptor=False)
+    return fsa
+
+
+def main():
+    out_dir = Path("data/lang")
+    lexicon_filename = out_dir / "lexicon.txt"
+    sil_phone = "SIL"
+    sil_prob = 0.5
+
+    lexicon = read_lexicon(lexicon_filename)
+    phones = get_phones(lexicon)
+    words = get_words(lexicon)
+
+    lexicon_disambig, max_disambig = add_disambig_symbols(lexicon)
+
+    for i in range(max_disambig + 1):
+        disambig = f"#{i}"
+        assert disambig not in phones
+        phones.append(f"#{i}")
+
+    assert "<eps>" not in phones
+    phones = ["<eps>"] + phones
+
+    assert "<eps>" not in words
+    assert "#0" not in words
+    assert "<s>" not in words
+    assert "</s>" not in words
+
+    words = ["<eps>"] + words + ["#0", "<s>", "</s>"]
+
+    phone2id = generate_id_map(phones)
+    word2id = generate_id_map(words)
+
+    write_mapping(out_dir / "phones.txt", phone2id)
+    write_mapping(out_dir / "words.txt", word2id)
+    write_lexicon(out_dir / "lexicon_disambig.txt", lexicon_disambig)
+
+    L = lexicon_to_fst(
+        lexicon,
+        phone2id=phone2id,
+        word2id=word2id,
+        sil_phone=sil_phone,
+        sil_prob=sil_prob,
+    )
+
+    L_disambig = lexicon_to_fst(
+        lexicon_disambig,
+        phone2id=phone2id,
+        word2id=word2id,
+        sil_phone=sil_phone,
+        sil_prob=sil_prob,
+    )
+
+    # TODO(fangjun): add self-loops to L_disambig
+    # whose ilabel is phone2id['#0'] and olable is word2id['#0']
+    # Need to implement it in k2
+
+    if False:
+        # Just for debugging, will remove it
+        torch.save(L.as_dict(), out_dir / "L.pt")
+        torch.save(L_disambig.as_dict(), out_dir / "L_disambig.pt")
+
+        L.labels_sym = k2.SymbolTable.from_file(out_dir / "phones.txt")
+        L.aux_labels_sym = k2.SymbolTable.from_file(out_dir / "words.txt")
+        L_disambig.labels_sym = L.labels_sym
+        L_disambig.aux_labels_sym = L.aux_labels_sym
+        L.draw(out_dir / "L.svg", title="L")
+        L_disambig.draw(out_dir / "L_disambig.svg", title="L_disambig")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/egs/librispeech/ASR/local/test_prepare_lang.py b/egs/librispeech/ASR/local/test_prepare_lang.py
new file mode 100755
index 000000000..f36ef55c6
--- /dev/null
+++ b/egs/librispeech/ASR/local/test_prepare_lang.py
@@ -0,0 +1,86 @@
+#!/usr/bin/env python3
+
+# Copyright (c)  2021  Xiaomi Corporation (authors: Fangjun Kuang)
+
+import os
+import tempfile
+
+import k2
+from prepare_lang import (
+    add_disambig_symbols,
+    generate_id_map,
+    get_phones,
+    get_words,
+    lexicon_to_fst,
+    read_lexicon,
+    write_lexicon,
+    write_mapping,
+)
+
+
+def generate_lexicon_file() -> str:
+    fd, filename = tempfile.mkstemp()
+    os.close(fd)
+    s = """
+    !SIL SIL
+    <SPOKEN_NOISE> SPN
+    <UNK> SPN
+    f f
+    a a
+    foo f o o
+    bar b a r
+    bark b a r k
+    food f o o d
+    food2 f o o d
+    fo  f o
+    """.strip()
+    with open(filename, "w") as f:
+        f.write(s)
+    return filename
+
+
+def test_read_lexicon(filename: str):
+    lexicon = read_lexicon(filename)
+    phones = get_phones(lexicon)
+    words = get_words(lexicon)
+    print(lexicon)
+    print(phones)
+    print(words)
+    lexicon_disambig, max_disambig = add_disambig_symbols(lexicon)
+    print(lexicon_disambig)
+    print("max disambig:", f"#{max_disambig}")
+
+    phones = ["<eps>", "SIL", "SPN"] + phones
+    for i in range(max_disambig + 1):
+        phones.append(f"#{i}")
+    words = ["<eps>"] + words
+
+    phone2id = generate_id_map(phones)
+    word2id = generate_id_map(words)
+
+    print(phone2id)
+    print(word2id)
+
+    write_mapping("phones.txt", phone2id)
+    write_mapping("words.txt", word2id)
+
+    write_lexicon("a.txt", lexicon)
+    write_lexicon("a_disambig.txt", lexicon_disambig)
+
+    fsa = lexicon_to_fst(lexicon, phone2id=phone2id, word2id=word2id)
+    fsa.labels_sym = k2.SymbolTable.from_file("phones.txt")
+    fsa.aux_labels_sym = k2.SymbolTable.from_file("words.txt")
+    fsa.draw("L.pdf", title="L")
+
+    fsa_disambig = lexicon_to_fst(
+        lexicon_disambig, phone2id=phone2id, word2id=word2id
+    )
+    fsa_disambig.labels_sym = k2.SymbolTable.from_file("phones.txt")
+    fsa_disambig.aux_labels_sym = k2.SymbolTable.from_file("words.txt")
+    fsa_disambig.draw("L_disambig.pdf", title="L_disambig")
+
+
+if __name__ == "__main__":
+    filename = generate_lexicon_file()
+    test_read_lexicon(filename)
+    os.remove(filename)