class RSpec::Core::Configuration

def expect_with(*frameworks)

end
custom_config.custom_setting = true
config.expect_with OtherExpectationFramework do |custom_config|

yield the `configuration` object if given a block:
If the module responds to `configuration`, `expect_with` will

## Configuration

appropriate modules.
RSpec will translate `:rspec`, `:minitest`, and `:test_unit` into the

config.expect_with OtherExpectationFramework
config.expect_with :rspec, :minitest
config.expect_with :minitest
config.expect_with :test_unit
config.expect_with :rspec

module, or any combination thereof:
`frameworks` can be `:rspec`, `:test_unit`, `:minitest`, a custom

group.
Sets the expectation framework module(s) to be included in each example 
def expect_with(*frameworks)
  modules = frameworks.map do |framework|
    case framework
    when Module
      framework
    when :rspec
      require 'rspec/expectations'
      self.expecting_with_rspec = true
      ::RSpec::Matchers
    when :test_unit
      require 'rspec/core/test_unit_assertions_adapter'
      ::RSpec::Core::TestUnitAssertionsAdapter
    when :minitest
      require 'rspec/core/minitest_assertions_adapter'
      ::RSpec::Core::MinitestAssertionsAdapter
    else
      raise ArgumentError, "#{framework.inspect} is not supported"
    end
  end
  if (modules - @expectation_frameworks).any?
    assert_no_example_groups_defined(:expect_with)
  end
  if block_given?
    raise "expect_with only accepts a block with a single argument. Call expect_with #{modules.length} times, once with each argument, instead." if modules.length > 1
    raise "#{modules.first} must respond to `configuration` so that expect_with can yield it." unless modules.first.respond_to?(:configuration)
    yield modules.first.configuration
  end
  @expectation_frameworks.push(*modules)
end