-- -fn x fn y — prefix-minus where one or both operands are known-arity
-- calls. Companion to wh-prefix-call (#332). Pre-fix `-dbl 5 dbl 3`
-- mis-parsed as `BinOp(-, Ref(dbl), Ref(5))` with `dbl 3` orphaned,
-- because parse_minus called parse_operand directly instead of the
-- parse_prefix_binop_operand helper. Post-fix, the natural shape
-- parses directly.
-- dbl x = x*2. Used so cross-engine answers are exact integers.
dbl x:n>n;ret *x 2
-- both-calls: subtract two calls. `-dbl 5 dbl 3` = 10 - 6 = 4.
both-calls a:n b:n>n;-dbl a dbl b
-- left-call: call on the left, literal on the right.
left-call a:n b:n>n;-dbl a b
-- right-call: literal on the left, call on the right.
right-call a:n b:n>n;-a dbl b
-- negate-call: unary negation of a call. `- dbl 5` = -10.
negate-call x:n>n;- dbl x
-- run: both-calls 5 3
-- out: 4
-- run: left-call 5 3
-- out: 7
-- run: right-call 10 3
-- out: 4
-- run: negate-call 5
-- out: -10