@Override
    protected void doExecute(ExecutionContext context) {
        FieldValue input = context.getCurrentValue();
        for (Expression expression : this) {
            context.setCurrentValue(input).execute(expression);
            if (context.getCurrentValue() != null)
                break; // value found
        }
    }
An expression which returns the value of the first of a list of subexpressions which
returns a non-null value.

Syntax: sub-expression1 || sub-expression2 || ...

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