def add_beam(beam)
      raise ArgumentError, "Invalid argument 'beam'. Expected Beam, got #{beam.class}." unless beam.is_a?(Beam)
      @beams << beam unless @beams.include?(beam)
    end
def add_beam(beam)
      raise ArgumentError, "Invalid argument 'beam'. Expected Beam, got #{beam.class}." unless beam.is_a?(Beam)
      @beams << beam unless @beams.include?(beam)
    end
Adds a Beam to this Plan.

 @param [Beam] beam a beam instance to be associated with this plan