:: deftheorem defines `hpartial12| PDIFF_5:def 29 :
for f being PartFunc of (REAL 3),REAL
for D being set st f is_hpartial_differentiable`12_on D holds
for b₃ being PartFunc of (REAL 3),REAL holds
( b₃ = f `hpartial12| D iff ( dom b₃ = D & ( for u being Element of REAL 3 st u in D holds
b₃ . u = hpartdiff12 (f,u) ) ) );