spec: slightly more realistic example for type assertions
spec: slightly more realistic example for type assertions

For #17428.

Change-Id: Ia902b50cf0c40e3c2167fb573a39d328331c38c7
Reviewed-on: https://go-review.googlesource.com/31449
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

polyverse-security/go,cloudflare/go,suncycheng/go,linux-on-ibm-z/go,golang/go,Cofyc/go,Cofyc/go,derekmarcotte/go,momchil-velikov/go,josharian/go,goodlang/good,golang/go,derekmarcotte/go,codestation/go,Cofyc/go,suncycheng/go,ImJasonH/go,polyverse-security/go,vsekhar/elastic-go,Cofyc/go,golang/go,derekmarcotte/go,akutz/go,cloudflare/go,momchil-velikov/go,josharian/go,derekmarcotte/go,suncycheng/go,goodlang/good,golang/go,momchil-velikov/go,christopher-henderson/Go,Cofyc/go,ImJasonH/go,polyverse-security/go,goodlang/good,nawawi/go,Cofyc/go,momchil-velikov/go,CAFxX/go,ImJasonH/go,CAFxX/go,vsekhar/elastic-go,cloudflare/go,nawawi/go,goodlang/good,Cofyc/go,akutz/go,vsekhar/elastic-go,suncycheng/go,golang/go,codestation/go,josharian/go,vsekhar/elastic-go,CAFxX/go,momchil-velikov/go,goodlang/good,Cofyc/go,akutz/go,momchil-velikov/go,derekmarcotte/go,ImJasonH/go,josharian/go,CAFxX/go,vsekhar/elastic-go,golang/go,CAFxX/go,linux-on-ibm-z/go,linux-on-ibm-z/go,codestation/go,nawawi/go,ImJasonH/go,cloudflare/go,josharian/go,nawawi/go,suncycheng/go,momchil-velikov/go,akutz/go,derekmarcotte/go,goodlang/good,linux-on-ibm-z/go,josharian/go,ImJasonH/go,nawawi/go,golang/go,suncycheng/go,ImJasonH/go,momchil-velikov/go,goodlang/good,linux-on-ibm-z/go,linux-on-ibm-z/go,vsekhar/elastic-go,cloudflare/go,nawawi/go,polyverse-security/go,polyverse-security/go,cloudflare/go,derekmarcotte/go,josharian/go,nawawi/go,codestation/go,suncycheng/go,derekmarcotte/go,codestation/go,goodlang/good,cloudflare/go,nawawi/go,polyverse-security/go,suncycheng/go,akutz/go,polyverse-security/go,josharian/go,CAFxX/go,codestation/go,CAFxX/go,vsekhar/elastic-go,codestation/go,ImJasonH/go,codestation/go,linux-on-ibm-z/go,akutz/go,cloudflare/go,akutz/go,linux-on-ibm-z/go,vsekhar/elastic-go,polyverse-security/go,golang/go,CAFxX/go,akutz/go