DEC

Subtract amount from a numeric variable (default 1). INC adds instead.

INC var [, amount] DEC var [, amount]

var - numeric variable amount - value to add or subtract (optional; default 1)

Example:

A = 10
DEC A
DEC A, 2
PRINT A

Source: docs/help/dec.txt inmmcore. On the computer, typeHELP DEC.