= Type extension
<Type extension> constructs a new <data type> from a base <data type> while retaining a specified relationship with it. In <Oberon>, a <record type> can extend another <record type> by adding fields while preserving its base fields. The language also relates corresponding pointer types and supplies <type guards> for accessing an extended value.<The Programming Language Oberon by Niklaus Wirth>[§§6.3–6.4, record extension and pointer types; §8.1, type guards]{ref}
For example, a base `Point` can contain coordinates `x,y`; an extending `ColoredPoint` retains those coordinates and adds `color`. Code using a permitted base interface can operate on the inherited coordinates, while access to `color` requires knowledge of the extension. This is a declared relationship in the <type system>, rather than an inference that two unrelated <record types> happen to have similarly named fields.
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