code: func (g *GeoM) Concat(other GeoM) {
  	a := (other.a_1+1)*(g.a_1+1) + other.b*g.c
  	b := (other.a_1+1)*g.b + other.b*(g.d_1+1)
  	tx := (other.a_1+1)*g.tx + other.b*g.ty + other.tx
  	c := other.c*(g.a_1+1) + (other.d_1+1)*g.c
  	d := other.c*g.b + (other.d_1+1)*(g.d_1+1)
  	ty := other.c*g.tx + (other.d_1+1)*g.ty + other.ty
  
  	g.a_1 = a - 1
  	g.b = b
  	g.c = c
  	g.d_1 = d - 1
  	g.tx = tx
  	g.ty = ty
  }
docs: // Concat multiplies a geometry matrix with the other geometry matrix.
  // This is same as muptiplying the matrix other and the matrix g in this order.