code: def is_parans_exp(istr):
      
      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
docs: Determines if an expression is a valid function "call"