aima/logic.py
def expr(s):
    """Create an Expr representing a logic expression by parsing the input
    string. Symbols and numbers are automatically converted to Exprs.
    In addition you can use alternative spellings of these operators:
      'x ==> y'   parses as   (x >> y)    # Implication
      'x <== y'   parses as   (x << y)    # Reverse implication
      'x <=> y'   parses as   (x % y)     # Logical equivalence
      'x =/= y'   parses as   (x ^ y)     # Logical disequality (xor)
    But BE CAREFUL; precedence of implication is wrong. expr('P & Q ==> R & S')
    is ((P & (Q >> R)) & S); so you must use expr('(P & Q) ==> (R & S)').
    >>> expr('P <=> Q(1)')
    (P <=> Q(1))
    >>> expr('P & Q | ~R(x, F(x))')
    ((P & Q) | ~R(x, F(x)))
    """
    if isinstance(s, Expr): return s
    if isnumber(s): return Expr(s)
    ## Replace the alternative spellings of operators with canonical spellings
    s = s.replace('==>', '>>').replace('<==', '<<')
    s = s.replace('<=>', '%').replace('=/=', '^')
    ## Replace a symbol or number, such as 'P' with 'Expr("P")'
    s = re.sub(r'([a-zA-Z0-9_.]+)', r'Expr("\1")', s)
    ## Now eval the string.  (A security hole; do not use with an adversary.)
    return eval(s, {'Expr':Expr})
def expr(s):
    """Create an Expr representing a logic expression by parsing the input
    string. Symbols and numbers are automatically converted to Exprs.
    In addition you can use alternative spellings of these operators:
      'x ==> y'   parses as   (x >> y)    # Implication
      'x <== y'   parses as   (x << y)    # Reverse implication
      'x <=> y'   parses as   (x % y)     # Logical equivalence
      'x =/= y'   parses as   (x ^ y)     # Logical disequality (xor)
    But BE CAREFUL; precedence of implication is wrong. expr('P & Q ==> R & S')
    is ((P & (Q >> R)) & S); so you must use expr('(P & Q) ==> (R & S)').
    >>> expr('P <=> Q(1)')
    (P <=> Q(1))
    >>> expr('P & Q | ~R(x, F(x))')
    ((P & Q) | ~R(x, F(x)))
    """
    if isinstance(s, Expr): return s
    if isnumber(s): return Expr(s)
    ## Replace the alternative spellings of operators with canonical spellings
    s = s.replace('==>', '>>').replace('<==', '<<')
    s = s.replace('<=>', '%').replace('=/=', '^')
    ## Replace a symbol or number, such as 'P' with 'Expr("P")'
    s = re.sub(r'([a-zA-Z0-9_.]+)', r'Expr("\1")', s)
    ## Now eval the string.  (A security hole; do not use with an adversary.)
    return eval(s, {'Expr':Expr})
Create an Expr representing a logic expression by parsing the input
    string. Symbols and numbers are automatically converted to Exprs.
    In addition you can use alternative spellings of these operators:
      'x ==> y'   parses as   (x >> y)    # Implication
      'x <== y'   parses as   (x << y)    # Reverse implication
      'x <=> y'   parses as   (x % y)     # Logical equivalence
      'x =/= y'   parses as   (x ^ y)     # Logical disequality (xor)
    But BE CAREFUL; precedence of implication is wrong. expr('P & Q ==> R & S')
    is ((P & (Q >> R)) & S); so you must use expr('(P & Q) ==> (R & S)').
    >>> expr('P <=> Q(1)')
    (P <=> Q(1))
    >>> expr('P & Q | ~R(x, F(x))')
    ((P & Q) | ~R(x, F(x)))