math.extras: adding all-removals

Author: mrjbq7

extra/math/extras/extras-tests.factor

@@ -127,3 +127,9 @@ tools.test ;
 { 1 } [ 11 13 stein ] unit-test
 { 2 } [ 14 52 stein ] unit-test
 { 7 } [ 14 7 stein ] unit-test
+
+{ { { 2 3 4 } { 1 3 4 } { 1 2 4 } { 1 2 3 } } } [ 1 { 1 2 3 4 } all-removals ] unit-test
+{ { { 3 4 } { 2 4 } { 2 3 } { 1 4 } { 1 3 } { 1 2 } } } [ 2 { 1 2 3 4 } all-removals ] unit-test
+{ { { 4 } { 3 } { 2 } { 1 } } } [ 3 { 1 2 3 4 } all-removals ] unit-test
+{ { { } } } [ 4 { 1 2 3 4 } all-removals ] unit-test
+{ { } } [ 5 { 1 2 3 4 } all-removals ] unit-test

extra/math/extras/extras.factor

@@ -410,3 +410,6 @@ M:: vose random* ( obj rnd -- elt )
     dup obj probs>> nth-unsafe { float } declare rnd random-unit* >=
     [ obj alias>> nth-unsafe { fixnum } declare ] unless
     obj items>> nth-unsafe ;
+
+:: all-removals ( n seq -- seqs )
+    seq length <iota> n over '[ _ swap diff seq nths ] map-combinations ;