;'< Tests for functionality "Evaluate Robin Expression (with literal)"
`sign` evaluates its sole argument to a number, then
evaluates to 0 if that number is 0, 1 if that number is positive, or
-1 if that number is negative.
| (sign 26)
= 1
| (sign 0)
= 0
| (sign (subtract 0 200))
= -1
`sign` expects a number.
| (sign #f)
? abort (expected-number #f)
| (sign (literal k))
? abort (expected-number k)
`sign` expects exactly one argument.
| (sign 100 200 300)
? abort (illegal-arguments (100 200 300))
| (sign)
? abort (illegal-arguments ())
'<