Teach "set" builtin to check for existence of variable at runtime.
Teach "set" builtin to check for existence of variable at runtime.

elves/elvish,CodyGuo/elvish,elves/elvish,elves/elvish,beni55/elvish,beni55/elvish,ke7ofi/elvish,elves/elvish,CodyGuo/elvish,elves/elvish,ke7ofi/elvish