def is_parans_exp(istr):
    """
    Determines if an expression is a valid function "call"
    """
    fxn = istr.split('(')[0]
    if not fxn.isalnum() and fxn != '(' or istr[-1] != ')':
        return False
    plevel = 1
    for c in '('.join(istr[:-1].split('(')[1:]):
        if c == '(':
            plevel += 1
        elif c == ')':
            plevel -= 1
        if plevel == 0:
            return False
    return True
