:: deftheorem defines trap SEMI_AF1:def 8 :
for SAS being Semi_Affine_Space
for a, b, c, d, o being Element of SAS holds
( trap a,b,c,d,o iff ( not o,a,c are_collinear & o,a,b are_collinear & o,c,d are_collinear & a,c // b,d ) );