I Want To . . .

Configure my . . .

Geometry

<geometry>
  ...
</geometry>

Use a variable

Variables must first be defined:

    <define name="my_variable" value="0.510"/>

And then they can be used by giving the variable name in square brackets:

    <define name="z_length" value="[trap_length]"/>

Use function evaluation

The variable defined in the first line is used in the function provided on the second line.

    <define name="trap_length" value="3.35e-3"/>
    <define name="r_length" value="{[trap_length] / sqrt(2.)}"/>

Any function that can be evaluated in ROOT's TFormula class can be understood by Kassiopeia's function evaluation.