Some ideas about negative conditionals

gistfile1.groovy

Object.metaClass.not_in = {obj ->
    !(delegate in obj)
}

boolean ifnot(boolean val, Closure closure) {
    if (!val) {
        closure.call()
    }
}

def value = 1
def valueList = [3,4,5]

ifnot(value in valueList) {
    println "Not in"
} 

if (value.not_in(valueList)) {
    println "Not in"
} else {
    println "in"
}