code: function(that) {
      if (this.sourceString !== that.sourceString) {
        throw errors.intervalSourcesDontMatch();
      }
      assert(this.startIdx >= that.startIdx && this.endIdx <= that.endIdx,
             'other interval does not cover this one');
      return new Interval(this.sourceString,
                          this.startIdx - that.startIdx,
                          this.endIdx - that.startIdx);
    }
docs: Returns a new Interval that has the same extent as this one, but which is relative to `that`, an Interval that fully covers this one.