code: function _filterSubjects(state, subjects, frame) {
    // filter subjects in @id order
    var rval = {};
    for(var i = 0; i < subjects.length; ++i) {
      var id = subjects[i];
      var subject = state.subjects[id];
      if(_filterSubject(subject, frame)) {
        rval[id] = subject;
      }
    }
    return rval;
  }
docs: Returns a map of all of the subjects that match a parsed frame.
  
  @param state the current framing state.
  @param subjects the set of subjects to filter.
  @param frame the parsed frame.
  
  @return all of the matched subjects.