Inherit callable behavior through delegation

Canonical source: protos/examples/objects/inherited-callability.protos at locked revision e99b712ecf658a527b1fe82c8beba2c1286d8dc9.

This executable example is rendered from the exact Protos source consumed by the website. Its program body is copied without modification.

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 *
 * Software distributed under the License is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for the specific language governing rights and limitations under the License.
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// `call` is an ordinary slot. A child can inherit it through delegation, and
// invocation still keeps the child as the receiver.
callable: {
    value: 1
    call: () => { value }
}

derived: callable {
    value: 9
}

print(derived())

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