Broken formatting in the description for +, -, *.

The type of this exception is not specified.

Crystal: the sample tests expect ArgumentError.

I suggest to change the estimated rank to 5 kyu.

• Random tests are very poor
• Output format is not specified
• Nothing is told about errors
• It may not be clear that "stack-based" means that the user has to build a stack from the list of operations, and not that the input is in postfix notation (the input is not a stringified stack of operands and operators)
• Interpreting Forth is very similar to implementing a postfix calculator; I wouldn't call it a duplicate, but it doesn't feel novel either