DEF MUX21
    PORT IN I0
    PORT IN I1
    PORT IN S
    PORT OUT Z

    NET A
    NET B
    NET C

    INST NAND1 NAND S  S  A
    INST NAND2 NAND S  I1 B
    INST NAND3 NAND I0 A  C
    INST NAND4 NAND B  C  Z
ENDDEF