Adding two 1N914 diodes and an AND gate ahead of the inverter create a two-input NAND gate. With the two diodes reversed and a 910 Ohm resistor removed, a NOR gate is created.

This CMOS two-input combination NAND/NOR gate is a three-input, fourpin logic gate. A p-channel enhancementtype MOSFET (Q1) and an n-channel enhancement-type MOSFET (Q4) form one complementary ...

The NAND gate is a circuit that outputs '0' when both inputs are '1', and '0' otherwise, so if you arrange the NAND gate as follows, the Invert circuit is completed.