Attempts to convert 'source' to the types that this generator can
            handle. The intention is to produce the set of targets can should be
            used when generator is run.
            only_one:   convert 'source' to only one of source types
                        if there's more that one possibility, report an
                        error.

            Returns a pair:
                consumed: all targets that can be consumed.

def convert_to_consumable_types (self, project, name, prop_set, sources, only_one=False):
        """ Attempts to convert 'source' to the types that this generator can
            handle. The intention is to produce the set of targets can should be
            used when generator is run.
            only_one:   convert 'source' to only one of source types
                        if there's more that one possibility, report an
                        error.

            Returns a pair:
                consumed: all targets that can be consumed.
        """
        if __debug__:
            from .targets import ProjectTarget
            assert isinstance(name, basestring) or name is None
            assert isinstance(project, ProjectTarget)
            assert isinstance(prop_set, property_set.PropertySet)
            assert is_iterable_typed(sources, virtual_target.VirtualTarget)
            assert isinstance(only_one, bool)
        consumed = []
        missing_types = []

        if len (sources) > 1:
            # Don't know how to handle several sources yet. Just try
            # to pass the request to other generator
            missing_types = self.source_types_

        else:
            (c, m) = self.consume_directly (sources [0])
            consumed += c
            missing_types += m

        # No need to search for transformation if
        # some source type has consumed source and
        # no more source types are needed.
        if only_one and consumed:
            missing_types = []

        #TODO: we should check that only one source type
        #if create of 'only_one' is true.
        # TODO: consider if consuned/bypassed separation should
        # be done by 'construct_types'.

        if missing_types:
            transformed = construct_types (project, name, missing_types, prop_set, sources)

            # Add targets of right type to 'consumed'. Add others to
            # 'bypassed'. The 'generators.construct' rule has done
            # its best to convert everything to the required type.
            # There's no need to rerun it on targets of different types.

            # NOTE: ignoring usage requirements
            for t in transformed[1]:
                if t.type() in missing_types:
                    consumed.append(t)

        consumed = unique(consumed)

        return consumed