Modifies the contents of a variable containing a numeric object
by subtracting one from its previous value.
Parameters:
symbol -
A symbol object identifying a variable in the current context. The
variable must contain a numeric object.
Returns:
The new value of the variable.
Modifies the contents of the variable identified by
symbol by decrementing by one the value it had
previously stored.
The variable binded to symbol is expected to
contain a numeric object.
