Please add a matcher alias for this:

``` ruby
alias_matcher :a_value_between, :be_between
```

...which allows the matcher to read correctly in composed matcher expressions:

``` ruby
expect(ages).to include( a_value_between(10, 20) )
```

It also makes the failure message read better.  You can add a spec to `spec/rspec/matchers/aliases_spec.rb`.