7 API upgrade guide

Updating to a newer minor version (second number) should normally be compatible without code changes. New arguments for existing functions are either added as overload or default argument. Functions to remove are flagged as obsolete with a hint for the replacement. In rare cases, the obolete attribute will throw an compalation error. All obsolete members will be removed during a major version change (first number). This migration guide contains the information that was previously stored in the obsolte attribute.