def variables(s):
    """Return a set of the variables in expression s.
    >>> ppset(variables(F(x, A, y)))
    set([x, y])
    >>> ppset(variables(F(G(x), z)))
    set([x, z])
    >>> ppset(variables(expr('F(x, x) & G(x, y) & H(y, z) & R(A, z, z)')))
    set([x, y, z])
    """
    result = set([])
    def walk(s):
        if is_variable(s):
            result.add(s)
        else:
            for arg in s.args:
                walk(arg)
    walk(s)
    return result
Return a set of the variables in expression s.
    >>> ppset(variables(F(x, A, y)))
    set([x, y])
    >>> ppset(variables(F(G(x), z)))
    set([x, z])
    >>> ppset(variables(expr('F(x, x) & G(x, y) & H(y, z) & R(A, z, z)')))
    set([x, y, z])