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# Checkpoint C

At this point you should understand:

- that load `'debug'` loads debug utilities
- the general idea of verb debugging
- how control words create control structures by grouping sentences into blocks
- what the T block test result is
- how the test result determines which B block to execute
- how the test result determines when control structure execution is finished

Check your understanding by doing the following exercises:

- debug step through your convert verb
- create a temporary script file and define a verb called `conv` that is similar to `convert`, but insists on a `'f'` argument to do the conversion to Fahrenheit and gives a string result indicating there was an error if the left argument is neither `'c'` nor `'f'`. Hint: use the control structure sketched out here:

    if. x. = 'c' do. ...
      elseif. x. = 'f' do. ...
      elseif. 1 do. 'left arg not c or f'
    end.

or try a `select.` structure.

- create a temporary script file and define a dyad called plus that adds its left argument to its right. But, if there is an error, it should give a string result. Hints: use dyad : `0; 4 plus 9` should return 13;`'a' plus 9` should return your error string (perhaps, `'there was an error'`); use a `try.` control structure to catch the error and give the string result.

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