theorem Th59:
  e in v.edgesOut() iff ex x being set st e DJoins v,x,G
proof
  hereby
    set x = (the_Target_of G).e;
    assume
A1: e in v.edgesOut();
    take x;
    (the_Source_of G).e = v by A1,Lm8;
    hence e DJoins v,x,G by A1;
  end;
  given x being set such that
A2: e DJoins v,x,G;
  e in the_Edges_of G & (the_Source_of G).e = v by A2;
  hence thesis by Lm8;
end;
