func parseImportDecl(tokens: ArraySlice<TokenSyntax>) throws -> (decl: ImportDecl, rest: ArraySlice<TokenSyntax>)? { var index = tokens.startIndex ... return (decl: ImportDecl(keywordIndex: keywordIndex - tokens.startIndex, nameIndex: nameIndex - tokens.startIndex, tokens: Array(tokens[..<index])), rest: tokens[index...]) } (edited)