Decrement

Decrement[a] subtracts 1 from a and returns the original value.

Attributes[Decrement] := {HoldFirst, Protected, ReadProtected}

Simple examples

In[1]:= toModify = 3
Out[1]= 3

In[2]:= Decrement[toModify]
Out[2]= 3

In[3]:= toModify
Out[3]= 2