PreDecrement
PreDecrement[a]
subtracts 1 from a
and returns the new value.
Attributes[PreDecrement] := {HoldFirst, Protected, ReadProtected}
Simple examples
In[1]:= toModify = 3 Out[1]= 3
In[2]:= PreDecrement[toModify] Out[2]= 2
In[3]:= toModify Out[3]= 2