ERROR: bad-ffi factor-word-name ;

: string>word ( string -- word )
    dup ":" split1 swap vocab-words at
    [ ]
    [ bad-ffi ] ?if ;

M: string selector-value string>word execute( -- result ) ;
M: string selector-value: string>word execute( input -- result ) ;
M: string selector-value:value: string>word execute( input input -- result ) ;
M: string selector-value:value:value: string>word execute( input input input -- result ) ;
M: string selector-value:value:value:value: string>word execute( input input input input -- result ) ;

----
example of use:

class String extends Object [
    |underlying|
    method underlying [ ^underlying ].
    method underlying: value [ underlying := value ].
    method length [ ^'sequences:length' value: underlying ].
].
(String new underlying: 'foo') length