This command fits a curve between two tangent lines and a known
external secant distance. It prompts for the P.I. and points on the
two tangent lines then the external distance.
[int on] Pick P.I. of
curve: pick a point
[nea on] Pick Point on 1st Tangent
Line: pick a point
[nea on] Pick Point on 2nd Tangent
Line: pick a point
External Distance
<50.00>: press Enter
Pulldown Menu Location: Draw > Arc
Keyboard Command: 2tanlex
Prerequisite: Tangent lines should be drawn before
execution