Add nfirst to combinators.lib and add seq>stack

extra/combinators/lib/lib-tests.factor

@@ -58,3 +58,5 @@ IN: temporary
         [ dup array? ] [ dup vector? ] [ dup float? ]
     } || nip
 ] unit-test
+
+[ 1 2 3 4 ] [ { 1 2 3 4 } 4 nfirst ] unit-test

extra/combinators/lib/lib.factor

@@ -67,6 +67,12 @@ MACRO: napply ( n -- )
 
 : map-with2 ( obj obj list quot -- newseq ) 2 map-withn ; inline
 
+MACRO: nfirst ( n -- )
+    [ [ swap nth ] curry [ keep ] curry ] map concat [ drop ] compose ;
+
+: seq>stack ( seq -- )
+    dup length nfirst ; inline
+
 ! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 
 : sigma ( seq quot -- n ) [ rot slip + ] curry 0 swap reduce ;