go - Implicit private function import? -

how main.go able use buildindexmapping?

the function not uppercased how publicly accessible within package , without explicit import of mapping.go?

main.go: https://github.com/blevesearch/beer-search/blob/721fab2198a52d919133df08058fde8043bab365/main.go#l55

mapping.go: https://github.com/blevesearch/beer-search/blob/721fab2198a52d919133df08058fde8043bab365/mapping.go#l19

both in same package. in go import packages, not files, private function available.