let AFV be WeakAffSegm; :: thesis: for a, b, c, d being Element of AFV st a,b // c,d holds
a,b // d,c
let a, b, c, d be Element of AFV; :: thesis: ( a,b // c,d implies a,b // d,c )
assume A1: a,b // c,d ; :: thesis: a,b // d,c
d,c // c,d by Def1;
hence a,b // d,c by A1, Def1; :: thesis: verum