Let's add support for

• standard arithmetic +, -, *, /, %
• comparisons ==, !=, <, <=, >, >=
• boolean &&, ||, may be ?:
• if possible: string repetition, perl-style x (to replace `SELECT REPEAT()` in test files)
• alternatively: functions, like instr, substr, repeat, replace, etc (to be added as needed)

This should work in let, if, while, $( ... )