-bool lex_next_is_number (struct lexer *, int n);
-double lex_next_number (struct lexer *, int n);
-bool lex_next_is_integer (struct lexer *, int n);
-long lex_next_integer (struct lexer *, int n);
-bool lex_next_is_string (struct lexer *, int n);
+bool lex_next_is_number (const struct lexer *, int n);
+double lex_next_number (const struct lexer *, int n);
+bool lex_next_is_integer (const struct lexer *, int n);
+long lex_next_integer (const struct lexer *, int n);
+bool lex_next_is_string (const struct lexer *, int n);