code: def indentation_step(node):
      
      r = find_root(node)
      # Collect all indentations into one set.
      all_indents = set(i.value for i in r.pre_order() if i.type == token.INDENT)
      if not all_indents:
          # nothing is indented anywhere, so we get to pick what we want
          return u"    " # four spaces is a popular convention
      else:
          return min(all_indents)
docs: Dirty little trick to get the difference between each indentation level
      Implemented by finding the shortest indentation string
      (technically, the "least" of all of the indentation strings, but
      tabs and spaces mixed won't get this far, so those are synonymous.)