Function SCIPgetExprPartialDiffNonlinear 

pub unsafe extern "C" fn SCIPgetExprPartialDiffNonlinear(
    scip: *mut SCIP,
    expr: *mut SCIP_EXPR,
    var: *mut SCIP_VAR,
) -> f64

returns the partial derivative of an expression w.r.t. a variable (or SCIP_INVALID if there was an evaluation error)

@see SCIPexprGetDerivative()