theorem :: BVFUNC_4:16

for Y being non empty set
for a, b being Function of Y,BOOLEAN
for G being Subset of (PARTITIONS Y)
for PA being a_partition of Y holds All ((a 'eqv' b),PA,G) = (All ((a 'imp' b),PA,G)) '&' (All ((b 'imp' a),PA,G))