The lexer is misunderstanding a combination of the . variable and the subtraction operator:
mob/verb/test()
. = .&(.-1)
Compiler error:
.4294967295: undefined type path
mob/verb/test()
. = .&(. -1)
No compiler error.
Looks like it's picking up .-1 as path lookup operator. As a type can't start with "-", this is invalid.