is pure: more background
is pure: more background

explain why the compiler cannot reliably infer purity on its own.
For issue #1231.

dmaestro/doc,stmuk/doc,jonathanstowe/doc,MasterDuke17/doc,perl6/doc,antquinonez/doc,gfldex/doc,tbrowder/doc,jonathanstowe/doc,antquinonez/doc,gfldex/doc,jonathanstowe/doc,dmaestro/doc,perl6/doc,dmaestro/doc,antquinonez/doc,tbrowder/doc,stmuk/doc,antquinonez/doc,MasterDuke17/doc,gfldex/doc,antquinonez/doc,dmaestro/doc,gfldex/doc,jonathanstowe/doc,jonathanstowe/doc,stmuk/doc,MasterDuke17/doc,perl6/doc,tbrowder/doc,stmuk/doc,perl6/doc,tbrowder/doc,MasterDuke17/doc,stmuk/doc,gfldex/doc,perl6/doc,dmaestro/doc