  We propose a simple criterion to identify when Nambu-Goldstone bosons (NGBs)
for different symmetries are redundant. It solves an old mystery why crystals
have phonons for spontaneously broken translations but no gapless excitations
for equally spontaneously broken rotations. Similarly for a superfluid, the NGB
for spontaneously broken Galilean symmetry is redundant with phonons. The most
nontrivial example is Tkachenko mode for a vortex lattice in a superfluid,
where phonons are redundant to the Tkachenko mode which is identified as the
Boboliubov mode.
