The A-median of triangle ABC is the cevian line through the midpoint of the side BC.

If the third optional argument M is given, the object returned a line segment AM where M is the midpoint of the side BC.

For a detailed description of the median mA, use the routine detail (i.e., detail(mA))

Note that the routine only works if the vertices of the triangle are known.

The command with(geometry,median) allows the use of the abbreviated form of this command.

assume that the names of the horizontal and vertical axes are _x and _y, respectively