code: function(fs1, fs2) {
          if(!fs2 || (fs2.length > fs1.length)) return false;
          for (var key in fs2) {
              var fact = fs2[key];
              if(!(fact.appearsIn(fs1))) {
                  return false;
              }
          }
          return true;
      }
docs: Checks if a set of facts is a subset of another set of facts.
  @param fs1 the superset
  @param fs2 the potential subset