Function SCIPcallExprMonotonicity 

pub unsafe extern "C" fn SCIPcallExprMonotonicity(
    scip: *mut SCIP,
    expr: *mut SCIP_EXPR,
    childidx: c_int,
    result: *mut SCIP_MONOTONE,
) -> SCIP_RETCODE

calls the monotonicity callback for an expression

@see SCIP_DECL_EXPRMONOTONICITY

Returns unknown monotonicity if callback not implemented.