Cactus thorns implement 'implementations', and other thorns can then depend on particular implementations being present. However, quite often those are updated, and incompatibilities occur. Usually this is resolved by the developer in both thorns - the implementing and the using thorn. However, a user might just update one of them, and Cactus would not catch this error. Thus, I suggest to think about, and implement how to:
specify implementation versions e.g. IMPLEMENTS: Cactus (4.0) The version string within () would have be sorted in an intelligent way to allow comparisons .
specify dependencies on particular versions of other implementations In particular I suggest the following dependencies: depends, and conflicts, both with comparisons < <= == >= > != e.g. DEPENDS: Cactus (>= 4.0) DEPENDS: BadImplementation (!=3.14) <-- this introduces a dependency CONFLICTS: BadImplementation (==3.14) <-- this doesn't introduce a depencency
 First the initial part of each string consisting entirely of non-digit characters is determined. These two parts (one of which may be empty) are compared lexically. If a difference is found it is returned. The lexical comparison is a comparison of ASCII values modified so that all the letters sort earlier than all the non-letters. Then the initial part of the remainder of each string which consists entirely of digit characters is determined. The numerical values of these two parts are compared, and any difference found is returned as the result of the comparison. For these purposes an empty string (which can only occur at the end of one or both version strings being compared) counts as zero. These two steps (comparing and removing initial non-digit strings and initial digit strings) are repeated until a difference is found or both strings are exhausted.