difficult to classify

References in classic literature ?
Rebecca in particular was so difficult to classify that Miss Dearborn at the end of a fortnight gave up the attempt altogether.
Despite this his music has a history of being difficult to classify - he combines world and ethnic musical influences from Eastern Europe, Africa and Celtic sounds with western classical techniques.
However difficult to classify, it is clearly a very intelligent and ambitious collection.
But sometimes it's very difficult to classify books into categories.
It seemed that the results themselves proved quite difficult to classify and the conclusions were not easily definable.
New challenges are complex, which means that it is difficult to classify conflicts and define which legal rules should apply to them, said Minister Czaputowicz.
As highlighted by the team's analysis and that of previous researchers 'RTEBC' (ready to eat breakfast cereals) and 'fruit juices' are items of consumption that have become difficult to classify as 'healthy' or 'unhealthy' as they can be an important source of nutrients for young children but also contain relatively high levels of free sugar.
Fortunately, the method is highly adjustable, and for data that are likely to be more or less difficult to classify, one can adjust the number of these hyperplane combinations.
Luke's Bottle Band music is difficult to classify, as it is written for the bottle, but the band also takes on a wide variety of genres of music.
The minister said these numbers are facing serious challenges and difficult environmental conditions affecting their access to good education and added that these challenges make it difficult to classify these migrants in terms of number, age and school stages they were in before migrating from their country.
"It is difficult to classify if someone has severe PCOS or a milder version, that doesn't really exist.
The texts gathered in these two collections are difficult to classify according to genre, but most of them record something out of the ordinary, from avenging ghosts to miraculous healings, strange animal behavior, journeys to hell, spirit possession, prophetic dreams, and exemplary filiality--to mention only a few of the typical topics.