# File lib/ruote/svc/dollar_sub.rb, line 191
      def extract_prefix(key)

        i = key.index(':')

        return [ 'f', key ] if not i
          # 'f' is the default prefix (field, not variable)

        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