@Override
    public ValueExpression setVariable(String variable, ValueExpression expression) {
        if (this.vars == null) {
            this.vars = new HashMap<>();
        }

        return this.vars.put(variable, expression);
    }
Utility class for wrapping a VariableMapper. Modifications occur to the internal Map instance.
The resolving occurs first against the internal Map instance and then against the wrapped VariableMapper
if the Map doesn't contain the requested ValueExpression.
primefaces-integration-tests/src/main/java/org/primefaces/integrationtests/common/VariableMapperWrapper.java