What does this log?

Console output

Why

`Hash.new([])` uses a single array object as the default for all missing keys. `h[:a] << 1` mutates this shared default (it does NOT assign a new value to `:a`). Both `:a` and `:b` return the same mutated default `[1, 2]`. Use `Hash.new { |h, k| h[k] = [] }` to create distinct arrays.