The Wallwright's Tally Stone

The wallwright of bitwick keeps the town wall in repair, and every cart that arrives brings a delivery slip scrawled by whoever loaded it. The guild will only carve a measure into the tally stone in one fixed form, so the slips must be rewritten before they are carved.

Measures use four units:

• 1 league = 4 rods
• 1 rod = 6 spans
• 1 span = 5 nails

Write canonicalTally(slip), which takes a string and returns a string.

Reading the slip. The slip's tokens are separated by one or more whitespace characters, and there may be leading or trailing whitespace. The slip may be empty or contain only whitespace. Otherwise it is made of:

• An optional first token minus. If present, it negates the *entire* total of everything after it. It can appear only as the first token, and at most once.
• Then zero or more terms. Each term is optionally preceded by the token less, which negates *only that one term*. The term itself is a count token followed by a unit token. A count is an optional - followed by one or more decimal digits. A unit token is one of league, leagues, rod, rods, span, spans, nail, nails — singular and plural mean the same thing in the input, whatever the count is.

The slip's value, in nails, is the sum of every term's count times its unit's size in nails, with a term's sign flipped if less precedes it, and the whole sum negated if the slip begins with minus. You may assume the slip is well formed as described.

Carving the stone. Return the canonical rendering of that value:

• If the value is zero, return exactly "0 nails".
• Otherwise, take the absolute value and break it into leagues, rods, spans and nails so that the count of nails is under 5, the count of spans is under 6, and the count of rods is under 4.
• Render each unit whose count is not zero, largest unit first, as the count, a single space, and the unit name — singular when the count is exactly 1, plural otherwise. Units with a count of zero are not rendered at all.
• Join those with single spaces. If the value is negative, put the token minus and a single space before the whole thing.

There is no other whitespace in the returned string.