Deleting the middle delimiter of let = in or if then else triggers Nonconvex Segment exception

[disconcision]

(Exception "Haz3lcore.Skel.Nonconvex_segment")

This only seems to occur:

1. when I try to delete (only a) "=" or "then" starting from complete syntax; note that if I delete a selection also containing any other non-whitespace token (part of a different form) to the left or the right of the =/then, it works fine.
2. when try to delete a selection containing both the first and second delimiter, e.g. "let ...=" or "if ... then"

I feel like there's one or more issues for this but I can't immediately find any.

@dm0n3y any feeling for how hard it would be to fix?