lexer (x1:x2:xs) | isPathSeparator x1, isPathSeparator x2 = SlashSlash : lexer xs
lexer (x1:x2:xs) | isPathSeparator x1, isPathSeparator x2 = SlashSlash : lexer xs
lexer (x1:x2:xs) | isPathSeparator x1, isPathSeparator x2 = SlashSlash : lexer xs