too difficult

References in classic literature ?
Dorothea's heart was full of something that she wanted to say, and yet the words were too difficult.
Matching instructional material to students' level of reading ability facilitates generalization to new material better than presenting material that is too difficult (Daly, Bonfigio, Mattson, Persampieri, & Foreman-Yates, 2005).
Because their auditory processing problems make the oral explanations and examples confusing, students often act as if their math assignments are much too difficult.
One possible reason is that the tools provided by the acquisition and technology community are too difficult to use.
It's not too difficult to guess the lyrics of the Owada songs "1-100," "start middle end," or "1234," which, with a wry wit, play with and off standard rock riffs and formulas, all delivered with deadpan precision spliced with the driving rhythms of rock 'n' roll.
In a sense, this is the classic long-term care admission: Family members have made heroic attempts to care for the patient at home, but the challenges have proven too difficult.
If an RTOS is too difficult to port to customer's proprietary hardware or if the API is unmanageable, evolution will make that RTOS extinct.