code: def find_val(self, eq, val):
          
          if eq not in ('f', 'g', 'q'):
              return
          elif eq in ('f', 'q'):
              key = 'unamex'
          elif eq == 'g':
              key = 'unamey'
          idx = 0
          for m, n in zip(self.system.varname.__dict__[key], self.__dict__[eq]):
              if n == val:
                  return m, idx
              idx += 1
          return
docs: Return the name of the equation having the given value