This paper firstly provides a re-appraisal of the development of techn
iques for inverting deduction, secondly introduces Mode-Directed Inver
se Entailment (MDIE) as a generalisation and enhancement of previous a
pproaches and thirdly describes an implementation of MDIE in the Progo
l system. Progol is implemented in C and available by anonymous ftp. T
he re-assessment of previous techniques in terms of inverse implicatio
n leads to new results for learning from positive data and inverting i
mplication between pairs of clauses.