Number building interface and api

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Given a method to check whether a DDC number is valid, we can provide a simple number building interface with this algorithm:

nextBuildableNotations(notation, maxDepth): // given a start notation, try to add at most maxDepth digits
  narrower=[]
  for digit in 0...9:
    build = notation.length === 3 ? notation + "." + suffix : notation.suffix
    if isValidNotation(build): // found a digit that can be added
      narrower.push(build)
    else if maxDepth > 0: // try adding multiple digits at once
      narrower.append(nextNotations(build, maxDepth - 1))  
  return narrower

This basically results in JSKOS API method narrower for the virtual hierarchy of all possibly composable DDC notations.

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Here is a possible Mockup. The building part should be collapsible. Clicking on the ↳ should truncate the current number.

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Another place where to put the selection of narrower classes could be the input field via typeahead (#62).