SHOW BEAM [PLANE|FIELD Kn] [First Last]
Show some sub-cube of the (up to 4-D) dirty Beam.
The 2nd argument indicates whether SHOW shows all Fields for a given
frequency plane (PLANE keyword), or all Frequency-dependent beams for a
given Field. The 2nd and 3rd arguments are optional if the beam is only
3-D, which occurs for a single field or a frequency-indepedent beam in
Mosaics (narrow band approximation).