class Predicate
  module Intersect
    include Expr

    def priority
      80
    end

    def identifier
      self[1]
    end

    def values
      self[2]
    end

    def free_variables
      @free_variables ||= identifier.free_variables
    end

    def constant_variables
      values.size == 1 ? free_variables : []
    end

  end
end