How to define in CFEngine 3 a class as the result of ORing several other classes

test.cf

body common control
{
   inputs => { "/var/cfengine/inputs/cfengine_stdlib.cf" };
   bundlesequence => { "test" };
}

bundle agent test
{
  vars:
      "pkg" slist => { "one", "two", "three" };
  classes:
      "one" expression => "any";
      # a is set to its previous value, ORed with the class indicated by $(pkg),
      # looping over all the values in @(pkg).
      "a" or => { "a", "$(pkg)" };
  reports:
    a::
      "a is set";
    !a::
      "a is not set";
}