Consider the following:

@Grab('org.codehaus.groovy:groovy-dateutil:2.5.18')
def date = new Date(), cal = date.toCalendar()

The groovy-dateutil module includes extension methods, including "toCalendar(Date)". As the script above is typed, it is common to see "toCalendar" indicated as unrecognized (underlined). The extension module cache needs to be updated when a library is grabbed.