theorem Th28: :: AFPROJ:28

for AS being AffinSpace
for X, Y being Subset of AS
for a being POINT of (IncProjSp_of AS)
for A being LINE of (IncProjSp_of AS) st a = LDir Y & [X,1] = A & Y is being_line & X is being_line holds
( a on A iff Y '||' X )