collapse(toStart) {
    if (!this.range) {
      return;
    }

    this.range = false;

    if (toStart) {
      this.path.steps = this.path.steps.concat(this.start.steps);
      this.path.terminal = this.start.terminal;
    } else {
      this.path.steps = this.path.steps.concat(this.end.steps);
      this.path.terminal = this.end.terminal;
    }
  }
Collapse a CFI Range to a single CFI Position
@param {boolean} [toStart=false]