rhombus takes three mandatory arguments and two
optional arguments.
A,B, two points.
a, a real number.
Optionally, varc, vard, two variable names.
rhombus(A,B,a ⟨,varc,vard ⟩)
returns and draws the rhombus ABCD, where a is the
counterclockwise angle from AB to AC.
If the arguments varc and vard are given, then
C and D will be assigned to them.
