public void remove(@NonNull final String var) {
        final VariableState state = variableStateMap.get(var);
        if (state == null) {
            return;
        }
        setFormula(var, null);
        recalculateDependencies(var);
        recalculate(var);

        if (state.isNeededBy.isEmpty()) {
            variableStateMap.remove(var);
        }
    }
Removes a variable from this map.
<br />
Note that the variable may still be represented in this Map after removal with an empty state.
This will be the case if another variable's formula depends on the removed variable.
main/src/main/java/cgeo/geocaching/utils/formulas/VariableMap.java