if (versionParameter.versionAll()) {
iterator = new FullContentIterator(_content);
} else if (versionParameter.versionChangesSince()) {
iterator = new ChangesSinceContentIterator(_content, ((RequestParameterVersionChanges)versionParameter).versionNumber());
} else if (versionParameter.versionCurrent()) {
iterator = new CurrentContentIterator(_content);
} else if (versionParameter.versionSingle()) {
iterator = new VersionContentIterator(_content, ((RequestParameterVersionSingle)versionParameter).versionNumber());
}
// may be unnecessary
assert(iterator != null);