The incircle ic of triangle T is the circle inscribed inside the triangle.

If the optional argument is given and is of the form 'centername' = cn where cn is name, cn will be the name of the center of ic.

For a detailed description of the incircle ic, use the routine detail (i.e., detail(ic))

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

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