I need to modify map elements while iterating a map, in my case subtract some elements (list). Like this:

```
def a = [
   1: [1, 2, 3],
   2: [3, 2, 4],
   3: [3, 2, 4],
   4: [5, 2, 1],
]
def b = [3]
println a
a.values().each{ tr ->
   tr = tr - b
}
println a

```

The `a` map has not changed. Result is:

```
[1:[1, 2, 3], 2:[3, 2, 4], 3:[5, 3, 1], 4:[5, 2, 1]]
[1:[1, 2, 3], 2:[3, 2, 4], 3:[5, 3, 1], 4:[5, 2, 1]]

```

However, I want the result to be `[1:[1, 2], 2:[2, 4], 3:[5, 1], 4:[5, 2, 1]]`. What I am doing wrong? Due to the fact that the initial map is rather big I do not want to construct another map with less elements (result map).