Three separate promises

# Three separate promises (owner, 2026-09-25):
#   mut   — the value may be changed          immut — it may not
#   no_shadow    — the name may not be shadowed; it means this variable all the way down
# Shadowing (`shadow 'a' := …`) makes a new variable under the old name, so it needs no `mut`.
MAIN {
    let immut 'a' := 1;
    shadow 'a' := 2;                  # an immut can be shadowed: nothing is changed
    let 'b' := 3;                    # no word: it may not change either
    let no_shadow mut 'total' := 0;        # changeable, and 'total' always means this one
    change 'total' = 'total' plus 'a' plus 'b';
    std::print.useful['a', " ", 'b', " ", 'total', newln];
}