ParamDims defined inside mutable and re-used parts of the ParamSpace dict lead to infinite iteration

This is not good. It can occur in such a scenario:

foo: &foo
  bar: spam
  fish: !pdim
    default: 42
    range: [10]
also_foo:
  <<: *foo

In ParamSpace, the individual ParamDims are probably detected multiple times, while at the same time having a link via the mutable dicts... I'm not sure what's wrong here.