backspace() {
    if (!this.isPenInBounds_()) {
      return;
    }

    // Check if a backspace can be done.
    if (this.col_ <= 0 && this.row_ <= 0) {
      return;
    }

    if (this.col_ <= 0) {
      // Move pen back a row.
      this.col_ = this.colCount_ - 1;
      this.row_--;
    } else {
      // Move pen back a column.
      this.col_--;
    }

    // Erase the character occupied at that position.
    this.memory_[this.row_][this.col_] = null;
  }
Erases a character from the buffer and moves the pen back.