http://groovy-lang.org/operators.html

overloading:
http://groovy-lang.org/operators.html#Operator-Overloading
http://groovy.jmiguel.eu/groovy.codehaus.org/Operator+Overloading.html
http://www.ibm.com/developerworks/ru/library/j-pg10255/

smooth operator : safe navigation
a?.size() // will not throw NPE, but returns null

spread:
def a = [ 'a', 'bb', 'c' ]
assert a*.size() == [ 1, 2, 1 ]

spaceship : uses the compareTo
assert 2 <=> 4 == -1
assert 4 <=> 4 == 0
assert 6 <=> 4 == 1

Groovy Truth:
* Boolean true
* Non-zero numbers
* Non-null references
* Non empty strings
* Non empty collections
* Regular expressions with a match

elvis:
println a ?: 'foo'  // the result is a, if a is "true" and 'foo' otherwise