How to specify a point

The coordinates of a point can be expressed using the following syntax:

1)      x,y

2)      @length<angle (from the previous point you move to a distance using an angle)

3)      @ x,y (from the previous point you move to a distance in the X axis and to another distance in the Y axis)

4)      @ (previous point)

5)      length (from the previous point you move to a distance using the current mouse position)

6)      Coordinate specified in a coordinate reference system different from the current one