@Override
    protected void doExecute(ExecutionContext context) {
        // Noop: Set the output execution value to the current execution value
    }
Returns the current execution value, that is the value passed to this expression.
Referring to this explicitly is useful e.g to concatenate it to some other string:
... | input foo . " " . _ | ...

@author bratseth
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/ExecutionValueExpression.java