docs: def check_valid_varname(varname,
                          custom_units,
                          custom_structs,
                          constants,
                          pos,
                          error_prefix="Variable name invalid.",
                          exc=None):
      """ Handle invalid variable names """
      exc = VariableDeclarationException if exc is None else exc
  
      valid_varname, msg = is_varname_valid(varname, custom_units, custom_structs, constants)
      if not valid_varname:
          raise exc(error_prefix + msg, pos)
  
      return True