Principia mathematica, by Alfred North Whitehead ... and Bertrand Russell.

sFCTION B] THE SUB-CLASSES OF A GIVEN CLASS 411
*60-61. 1- t"a CCl ex'a [*37-61 *51-12 *60-6]
*60-62. F x, y Ea.:). t'xvtECl ex 'a [*60i6344]
*60-7. 1-. Cl'a e Cls2
Dem.
F *60-2.)F:/3EcC1l'a.r.fl Cao.
[*22-1.*20t13] (O*.=
[*10-5],
F. (1).- *60 2.- (*60-03). ) F Prop
*60-71. F.Cls2 =Cl'ICls [(*60-03)]
*60-72. F.ClS3 =Cl6C]S,2 [(*60-04)]