Returns the properties which specify implicit include paths to
            generated headers. This traverses all targets in this subvariant,
            and subvariants referred by <implcit-dependecy>properties.
            For all targets which are of type 'target-type' (or for all targets,
            if 'target_type' is not specified), the result will contain
            <$(feature)>path-to-that-target.

def implicit_includes (self, feature, target_type):
        """ Returns the properties which specify implicit include paths to
            generated headers. This traverses all targets in this subvariant,
            and subvariants referred by <implcit-dependecy>properties.
            For all targets which are of type 'target-type' (or for all targets,
            if 'target_type' is not specified), the result will contain
            <$(feature)>path-to-that-target.
        """
        assert isinstance(feature, basestring)
        assert isinstance(target_type, basestring)
        if not target_type:
            key = feature
        else:
            key = feature + "-" + target_type


        result = self.implicit_includes_cache_.get(key)
        if not result:
            target_paths = self.all_target_directories(target_type)
            target_paths = unique(target_paths)
            result = ["<%s>%s" % (feature, p) for p in target_paths]
            self.implicit_includes_cache_[key] = result

        return result