# File lib/ruote/util/dollar.rb, line 136
    def extract_prefix (key)

      i = key.index(':')

      return [ @default_prefix, key ] if not i

      pr = key[0..i-1] # until ':'
      pr = pr[0, 2] # the first two chars

      pr = pr[0, 1] unless (pr == 'vf') or (pr == 'fv')

      [ pr, key[i+1..-1] ]
    end