This intention changes a library method contract. Method contract has the following syntax:
contract ::= (clause ';')* clause
clause ::= args '->' effect
args ::= ((arg ',')* arg )?
arg ::= value-constraint
value-constraint ::= '_' | 'null' | '!null' | 'false' | 'true'
effect ::= value-constraint | 'fail' | 'new' | 'this' | 'param' number
number ::= [1-9] [0-9]*

The constraints denote the following:

Examples:

@Contract("_, null -> null") - method returns null if its second argument is null
@Contract("_, null -> null; _, !null -> !null") - method returns null if its second argument is null and not-null otherwise
@Contract("true -> fail") - a typical assertFalse method which throws an exception if true is passed to it