<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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