One obvious way to build such knowledge would be to prepare a systematic programme of regular research. Desirable as this might be, it is not the aim of this note to advocate such programmes . What is advocated is more realistic : that each new piece of information be seen and evaluated as part of a matrix of knowledge; that past findings be remembered and that present studies be conducted with some regard to possible future needs. However, in practice, even the small effort required to accumulate knowledge in this way is but rarely made. This is illustrated by an undramatic and therefore fairly typical case history.