2.10. The test_value element

A test_value element information item (referred to in this specification as a test_value element) is an element in the CellML namespace with a local name equal to test_value, which appears as a child of a reset element.

1. A test_value element MUST contain exactly one math element child.

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The test_value (like the reset_value) block takes a slightly different kind of MathML statement to those in the component elements. Because the the left hand side of the equation has already been effectively defined by specifying the test_variable, the math child of a test_value needs only to specify the right hand side. This means that the normal opening block of <apply><eq/><ci>left_hand_side_variable</ci> ... </apply> is omitted:

<reset variable="position" test_variable="height_above_the_floor_in_metres" order="1"/>
  <test_value>
    <math>
        <cn cellml:units="metre">1.0</cn>
    </math>
  </test_value>
  <reset_value>
    <math>
      <apply><plus/>
        <apply><abs/>
          <apply><minus/>
            <cn cellml:units="metre">1</cn>
            <ci>position</ci>
          </apply>
        </apply>
        <cn cellml:units="metre">1.0</cn>
      </apply>
    </math>
  </reset_value>
</reset>

In the above example, the position of a ball above a 1 metre high table is maintained, as every time it appears to drop below the table-level datum of position=1.0 it is reset to a “bounced” value greater than 1.0 instead.

For the reset to be useful, the equations it contains need to make sense. Be sure to check that the units that you’re using within the test_value block match those of the test_variable against which it will be compared. In this example the reset_value (the new position of the ball over the table) must be in metres to match the variable's units, as with the test_value which must match the units of the test_variable. A mismatch of units here doesn’t mean you have an invalid CellML 2.0 model, but it does mean that it may not behave in the way you are expecting!