seg.spad line 221 [edit on github]
This domain is used to provide the function argument syntax v=a..b. This is used, for example, by the top-level draw functions.
equation(v, a..b) creates a segment binding value with variable v and segment a..b. Note that the interpreter parses v=a..b to this form.
segment(segb) returns the segment from the right hand side of the SegmentBinding. For example, if segb is v=a..b, then segment(segb) returns a..b.
variable(segb) returns the variable from the left hand side of the SegmentBinding. For example, if segb is v=a..b, then variable(segb) returns v.