let G be Go-board; :: thesis: for i1, j1, i2, j2 being Nat st 1 <= i1 & i1 <= len G & 1 <= j1 & j1 + 1 <= width G & 1 <= i2 & i2 <= len G & 1 <= j2 & j2 + 1 <= width G & LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1)))) meets LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1)))) & not ( j1 = j2 & LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1)))) = LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1)))) ) & not ( j1 = j2 + 1 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1))))) = {(G * (i1,j1))} ) holds
( j1 + 1 = j2 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1))))) = {(G * (i2,j2))} )

let i1, j1, i2, j2 be Nat; :: thesis: ( 1 <= i1 & i1 <= len G & 1 <= j1 & j1 + 1 <= width G & 1 <= i2 & i2 <= len G & 1 <= j2 & j2 + 1 <= width G & LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1)))) meets LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1)))) & not ( j1 = j2 & LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1)))) = LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1)))) ) & not ( j1 = j2 + 1 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1))))) = {(G * (i1,j1))} ) implies ( j1 + 1 = j2 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1))))) = {(G * (i2,j2))} ) )
assume that
A1: ( 1 <= i1 & i1 <= len G ) and
A2: 1 <= j1 and
A3: j1 + 1 <= width G and
A4: ( 1 <= i2 & i2 <= len G ) and
A5: 1 <= j2 and
A6: j2 + 1 <= width G and
A7: LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1)))) meets LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1)))) ; :: thesis: ( ( j1 = j2 & LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1)))) = LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1)))) ) or ( j1 = j2 + 1 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1))))) = {(G * (i1,j1))} ) or ( j1 + 1 = j2 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1))))) = {(G * (i2,j2))} ) )
A8: i1 = i2 by A1, A2, A3, A4, A5, A6, A7, Th19;
A9: (j1 + 1) + 1 = j1 + (1 + 1) ;
A10: (j2 + 1) + 1 = j2 + (1 + 1) ;
A11: ( |.(j1 - j2).| = 0 or |.(j1 - j2).| = 1 ) by A1, A2, A3, A4, A5, A6, A7, Th19, NAT_1:25;
per cases ( j1 = j2 or j1 = j2 + 1 or j1 + 1 = j2 ) by ;
case j1 = j2 ; :: thesis: LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1)))) = LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1))))
hence LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1)))) = LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1)))) by A8; :: thesis: verum
end;
case j1 = j2 + 1 ; :: thesis: (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1))))) = {(G * (i1,j1))}
hence (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1))))) = {(G * (i1,j1))} by A1, A3, A5, A8, A10, Th13; :: thesis: verum
end;
case j1 + 1 = j2 ; :: thesis: (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1))))) = {(G * (i2,j2))}
hence (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * (i2,(j2 + 1))))) = {(G * (i2,j2))} by A1, A2, A6, A8, A9, Th13; :: thesis: verum
end;
end;