now

per cases ( e in the_Edges_of G or not e in the_Edges_of G ) ;

suppose e in the_Edges_of G ; :: thesis: G .labelEdge (e,x) is non-multi

then G .labelEdge (e,x) = G .set (ELabelSelector,((the_ELabel_of G) +* (e .--> x))) by Def21;
hence G .labelEdge (e,x) is non-multi ; :: thesis: verum

end;

suppose not e in the_Edges_of G ; :: thesis: G .labelEdge (e,x) is non-multi

hence G .labelEdge (e,x) is non-multi by Def21; :: thesis: verum

end;

end;

end;

hence G .labelEdge (e,x) is non-multi ; :: thesis: verum