code: function(matrix) {
        return Matrix(this.a * matrix.a + this.c * matrix.b, this.b * matrix.a + this.d * matrix.b, this.a * matrix.c + this.c * matrix.d, this.b * matrix.c + this.d * matrix.d, this.a * matrix.tx + this.c * matrix.ty + this.tx, this.b * matrix.tx + this.d * matrix.ty + this.ty);
      }
docs: Returns the result of this matrix multiplied by another matrix
  combining the geometric effects of the two. In mathematical terms,
  concatenating two matrixes is the same as combining them using matrix multiplication.
  If this matrix is A and the matrix passed in is B, the resulting matrix is A x B
  http://mathworld.wolfram.com/MatrixMultiplication.html
  @name concat
  @methodOf Matrix#
  @param {Matrix} matrix The matrix to multiply this matrix by.
  @returns {Matrix} The result of the matrix multiplication, a new matrix.