theorem
  M,v |= (p => r) => ((q => r) => (p 'or' q => r)) & M |= (p => r) => ((
  q => r) => (p 'or' q => r))
proof
  now
    let v;
    now
      assume
A1:   M,v |= p => r;
      now
        assume M,v |= q => r;
        then M,v |= p or M,v |= q implies M,v |= r by A1,ZF_MODEL:18;
        then M,v |= p 'or' q implies M,v |= r by ZF_MODEL:17;
        hence M,v |= p 'or' q => r by ZF_MODEL:18;
      end;
      hence M,v |= (q => r) => (p 'or' q => r) by ZF_MODEL:18;
    end;
    hence M,v |= (p => r) => ((q => r) => (p 'or' q => r)) by ZF_MODEL:18;
  end;
  hence thesis;
end;
