Get all variables that have never been written to.

        :return: A list of variables that are never written to.

def input_variables(self, exclude_specials=True):
        """
        Get all variables that have never been written to.

        :return: A list of variables that are never written to.
        """

        def has_write_access(accesses):
            return any(acc for acc in accesses if acc.access_type == 'write')

        def has_read_access(accesses):
            return any(acc for acc in accesses if acc.access_type == 'read')

        input_variables = [ ]

        for variable, accesses in self._variable_accesses.items():
            if not has_write_access(accesses) and has_read_access(accesses):
                if not exclude_specials or not variable.category:
                    input_variables.append(variable)

        return input_variables