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