Untype is a special type-system result meaning that no sensible type can be computed for an expression. A typeOf function returns the string "untype" when neither an obviously correct type nor a useful best-effort type exists, allowing callers to handle the failure explicitly. section 13.1 section 13.4
Untype should not spread blindly through an expression tree. Best-effort types and untype can bubble from subexpressions to their ancestors, so type computation should continue trying to produce a useful type and preserve the root problem’s location. section 13.1 section 13.4
