GENERIC: free-vars* ( form -- )

: free-vars ( form -- vars )
    [ free-vars* ] { } make prune ;

: add-if-free ( object -- )
    {
        { [ dup local-writer? ] [ "local-reader" word-prop , ] }
        { [ dup lexical? ] [ , ] }
        { [ dup quote? ] [ local>> , ] }
        { [ t ] [ free-vars* ] }
    } cond ;

M: object free-vars* drop ;

M: quotation free-vars* [ add-if-free ] each ;

M: lambda free-vars*
    [ vars>> ] [ body>> ] bi free-vars swap diff % ;