IsNearfield( nr )  
Let (N,+,cdot) be a near-ring with zero 0 and denote by N^{*} the set N - {0}. N is a nearfield if (N^{*},cdot) is a group.
The function IsNearfield tests if nr has an identity and 
if every non-zero element has a multiplicative inverse and returns
the according value true or false.
gap> IsNearfield( LibraryNearring( "V4", 16 ) ); true
GAP 3.4.4