The hidden rune

Problem

No spell in Valdoria is ever written in plain sight: it is hidden inside a long scroll, among other letters. The apprentice gets a spell and a scroll, and wants to know if the spell is in there.

Write a function that takes spell and scroll, two texts of lowercase letters with no spaces, and returns true if you can pick letters from the scroll, left to right, until you read the whole spell in order, and false otherwise. The letters don't have to be next to each other, but each letter of the scroll can be used only once: "ooo" is not in "moonlit", which only has two o's.

For example, "sun" is hidden in "assuming": the s, the u and the n show up in that order. "nus" is not, because after the n no u is left. An empty spell is always hidden, even when the scroll is empty. A spell with letters is never hidden in an empty scroll.

Examples

• The one in the example

  "sun", "assuming" → true

• The letters backwards

  "nus", "assuming" → false

• All next to each other

  "fire", "fire" → true

• One letter is missing

  "light", "lift" → false

• Scroll left over at the end

  "arm", "farmhouse" → true

Besides these, the challenge has hidden tests that are revealed when you submit your solution.

You start with this

def is_hidden(spell, scroll):
    pass

function isHidden(spell, scroll) {
}