Strict knowledge of subset of properties involved in a userdefined script

Hello,
I've a QScriptvalue m_ParametersObject containing a set of properties/double P1,P2...Pn
I want the user to define formula in a QTextEdit where a formula is a relation between a choice of parametersex: Let the parametersObject contain p1..p5.
The user defines the followinf formula in a textEdit : "p5 = p2+p4"
I need to know that p2 and p4 are involved in the forumla and that p1 and p3 are not involved.@
void AddFormulaDialog::formulaChanged() // textEdit slot
{
QString formula = ui>textEdit>toPlainText();
QScriptEngine * engine = m_ParametersObject>engine();
// m_ParametersObject is a QScriptValue, with parameters as properties p1, p2...pn
QScriptContext * context = engine>pushContext();
context>setActivationObject(*m_ParametersObject);
QScriptValue res = engine>evaluate(formula);QPushButton * okButton = ui>buttonBox>button(QDialogButtonBox::Ok); okButton>setEnabled(!res.isError()); engine>popContext();
}
@With this, I can get the analytical formula and the evaluation, but not the topological information: I don't know which parameters are actually involved int the relation.
QScriptValueIterator doesn't help here:
QScriptValueIterator it(*m_ParametersObject) gives me all properties of the context (not only of the formula)
QScriptValueIterator it(res) gives nothing.Is there a way (even completely different), to strictly retrieve the set of properties involved in the interactively user defined formula?
Note: I'm new to QtScript
Thank you in advance
Laurent