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 ::= 'any' | 'null' | '!null' | 'false' | 'true'
effect ::= value-constraint | 'fail' | 'exit'

The constraints denote the following:

- _ - any value
- null - null value
- !null - a value statically proved to be not-null
- true - true boolean value
- false - false boolean value
- fail - the method throws exception, if the arguments satisfy argument constraints
- exit - the method terminates the current process, if the arguments satisfy argument constraints

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