Access to query argument in groovy scripts?

I have written some scripts that mangle certain HTTP headers to inject different HTTP headers depending on the target address. I am able to get hold of HTTP headers like this:

  def request = testStep.httpRequest
  def headers = request.requestHeaders
  headers['applicationid'] = ['foobar']

Is it also possible to get access to individual query parameters and read and/or modify them? If so, how would that look like?

I fancy there must be something like:

def queryArg = request.???
def filter = queryArgs['filter']

to access e.g. the ...?filter=foobar&...  query argument.

What would that ??? in the above code snippet be?

Or would one have to do string manipulations on the request's URL?