How much code do you tend to write before you test

sum
    "sum up all elements.
     This is implemented using a variant of the normal inject:into: pattern.
     The reason for this is that it is not known whether we are dealing with number
     (i.e. if 0 is a good initial value for the sum).
     Consider a collection of measurement or physical objects, 0 would be the unitless
     value and would not be appropriate to add with the unit-ed objects."

    | sum sample |

    sample := self anElement.
    sum := self inject: sample into: [:accum :each | accum + each].
    ^ sum - sample.

    "
     TestCase should: [ { } sum ] raise:Error.
     TestCase should: [ ''  sum ] raise:Error.

     TestCase assert: ( { 1 } sum = 1 ).
     TestCase assert: ( { 1. 2. 3. 4. } sum = 10 ).
     TestCase assert: ( (1 to:10) sum = 55 ).
     TestCase assert: ( 'abc' asByteArray sum = 294 ).

     TestCase assert: ( { 10 +/- 2.
                          20 +/- 4.
                         100 +/- 10 } sum = (130 +/- 16) ).

     TestCase assert: ( { (1 / 9).
                          (1 / 7).
                        } sum = (16 / 63) ).
    "