code: func (m *Mat4x2) Set(row, col int, value float64) {
  	m[col*4+row] = value
  }
docs: // Set sets the corresponding matrix element at the given row and column.
  // This has a pointer receiver because it mutates the matrix.
  //
  // This method is garbage-in garbage-out. For instance, on a Mat4 asking for
  // Set(5,0,val) will work just like Set(1,1,val). Or it may panic if it's out of bounds.