def matchPreviousLiteral(expr):
    """
    Helper to define an expression that is indirectly defined from
    the tokens matched in a previous expression, that is, it looks
    for a 'repeat' of a previous expression.  For example::
        first = Word(nums)
        second = matchPreviousLiteral(first)
        matchExpr = first + ":" + second
    will match C{"1:1"}, but not C{"1:2"}.  Because this matches a
    previous literal, will also match the leading C{"1:1"} in C{"1:10"}.
    If this is not desired, use C{matchPreviousExpr}.
    Do I{not} use with packrat parsing enabled.
    """
    rep = Forward()
    def copyTokenToRepeater(s,l,t):
        if t:
            if len(t) == 1:
                rep << t[0]
            else:
                # flatten t tokens
                tflat = _flatten(t.asList())
                rep << And(Literal(tt) for tt in tflat)
        else:
            rep << Empty()
    expr.addParseAction(copyTokenToRepeater, callDuringTry=True)
    rep.setName('(prev) ' + _ustr(expr))
    return rep

    Helper to define an expression that is indirectly defined from
    the tokens matched in a previous expression, that is, it looks
    for a 'repeat' of a previous expression.  For example::
        first = Word(nums)
        second = matchPreviousLiteral(first)
        matchExpr = first + ":" + second
    will match C{"1:1"}, but not C{"1:2"}.  Because this matches a
    previous literal, will also match the leading C{"1:1"} in C{"1:10"}.
    If this is not desired, use C{matchPreviousExpr}.
    Do I{not} use with packrat parsing enabled.
    
src/pkg_resources/_vendor/pyparsing.py