[replace].
'Command examples:       CHECK|: replace.'.
'                        CHECK|: replace h#3.'.
'                        CHECK|: replace c#1: X+0.'.
'                        CHECK|: replace h#7: element(_,_) by 0.'.
'                        CHECK|: replace c#4: X+Y by Y+X using commut.'.
'                        CHECK|: replace all: a1+0 by a1 using arith.'.
''.
'The "replace" command may be used to replace a subexpression of one of'.
'the current hypotheses or conclusions by another expression, provided'.
'that the replacement may be justified by one of the replacement rules in'.
'the rules database.  The user is prompted for patterns for the appropriate'.
'expressions and may supply the name of the rule to be used.'.
''.
'If "replace all: ..." is used and a suitable substitution is found, all'.
'occurrences of the chosen (fully-instantiated) expression are replaced'.
'by the new expression, in every hypothesis and conclusion.'.
