Function +=

+= symbol value

Modifies the value stored in a variable by adding it a value.

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 added to symbol.

Returns:

The new value of the variable.

Modifies the contents of the variable identified by symbol by adding value to the value it had previously stored. The variable binded to symbol is expected to contain a numeric object.

The += is a short way to write

    set! symbol [+ $symbol $value]