The Apothecary's Rune-Wands

The apothecary of bitwick will not shelve a rune-wand until the counting-stone has read it. Each wand carries a single line of runes, read left to right; a line that breaks the craft's rules is faulty and the wand is worth nothing.

You are given line, a string. Split it on whitespace to get the runes (leading, trailing and repeated whitespace are ignored; a line with no runes at all is possible). Write wandValue(line), which returns the wand's value as an integer, or null in TypeScript / None in Python if the line is faulty.

## The runes

The stone keeps a stack of integers, empty at the start.

• A number is an optional - followed by one or more digits (7, -3, 007). It pushes its value.
• +, -, * each pop two values. Calling the deeper one a and the upper one b, they push a + b, a - b, a * b respectively.
• dup pushes a copy of the top value.
• swap exchanges the top two values.
• drop removes the top value.
• rep pops one value n, then runs the block that immediately follows it exactly n times, one run after another. If n is zero or negative the block is not run at all. Blocks may contain further reps.
• A block is the rune [, then zero or more runes, then its matching ].

## Faulty lines

The shape of the line is judged over every rune in it, whether or not that rune is ever reached. A line is faulty if any of these hold:

• some rune is not a number, not one of + - * dup swap drop rep, and not [ or ];
• a [ appears anywhere other than immediately after a rep;
• a rep is not immediately followed by a [;
• the brackets are not balanced and properly nested.

Otherwise the line is run. It is also faulty if:

• any rune needs more values than the stack currently holds (including rep needing its count);
• after every rune has been run, the stack does not hold exactly one value.

When the line is not faulty, the single value left on the stack is the result.