Fix the //= operator.

Closes #62.

tests/all.test

@@ -475,10 +475,9 @@ def inc(x): x |= .+1; inc(.[].a)
 [{"foo":[1,2], "bar": 42}, {"foo":[1], "bar": null}, {"foo":[null,false,3], "bar": 18}, {"foo":[], "bar":42}, {"foo": [null,false,null], "bar": 41}]
 [[1,2], [1], [3], [42], [41]]
 
-# FIXME: behaviour of update operators
-# .[] //= .[0]
-# ["hello",true,false,[false],null]
-# ["hello",true,"hello",[false],"hello"]
+.[] //= .[0]
+["hello",true,false,[false],null]
+["hello",true,"hello",[false],"hello"]
 
 .[] | [.[0] and .[1], .[0] or .[1]]
 [[true,[]], [false,1], [42,null], [null,false]]