A binding shadows a command

let dd = … shadows the command dd lexically, at every scope, and ^dd reaches the binary on PATH. Resolution of a bare head is one lexical question, bindings first, and no host state enters it.

What was decided

• No refusal. The session-scope check — a let or recursive definition naming an executable on the effective PATH was refused before its right-hand side ran — is gone, with its hint. Every binder now behaves as nested lets and lambda parameters always did.
• Nothing else changes. The checker resolves a head against bindings before commands, the runtime’s resolve does the same, and the elaborator’s is_bound already chose App(Force) for a bound head. The refusal was the lone place where the host’s PATH was consulted to decide whether a binding was legal.
• ^name is the escape, as it has been since an-external-is-a-byte-operation.

Why

A script’s meaning must not depend on its host. The old check made the same source bind on one machine and be refused on another, and again on the same machine after an install; it fired on let b where a home directory’s bin held a b. The source said “machine-dependent” of itself as a known weakness. A static rule that never reads the filesystem has none of these faults, and a bound name that hides a command is visible in the source.

Two phases no longer reason about one name: the decision was lexical (the elaborator) but the denial was dynamic (I/O), and the depth proxy (at_session_scope) that scoped the denial is not needed either.

What it costs

A session binding can hide a command the author forgot existed, such as let test = … silently replacing test. Reading $test still shows the binding, explain name reports what it shadows, and ^test names the command. Hiding a prelude function or builtin was already legal.

Running such a binding as a program is refused by T0072 (HeadBoundToValue) for a bare head, $name args and !$name alike, so the user is told of the binding rather than of the ! operator they never wrote. Its hint names $name as the value and offers ^name should a command have been meant; it never consults PATH to say whether one exists.