In particular, Hawking and Ellis show that singularities and black holes arise in a large class of plausible solutions. He warned that although this book is self-contained, it is more suitable for specialists rather than new students as it is heavy-going and contains no exercises. He noted that despite the authors' attempt at a rigorous treatment, certain technical terms, such as Lie groups, are used but never explained, and that modern coordinate-free methods are introduced, but not used effectively.[4]