ConseqConv.DEPTH_CONSEQ

STRUCTURE

SYNOPSIS

Applies a consequence conversion repeatedly to all the sub-terms of a term, in top-down order.

DESCRIPTION

DEPTH_CONSEQ_CONV c tm tries to apply the given conversion at toplevel. If this fails, it breaks the term tm down into boolean subterms. It can break up the following operators: /\, \/, ~, ==> and quantification. Then it applies the directed consequence conversion c to terms and iterates. Finally, it puts everything together again.

Notice that some operators switch the direction that is passed to c, e.g. to strengthen a term ~t, DEPTH_CONSEQ_CONV tries to weaken t.

EXAMPLE

Consider the expression FEVERY P (f |+ (x1, y1) |+ (x2,y2)). It states that all elements of the finite map f |+ (x1, y1) |+ (x2, y2) satisfy the predicate P. However, the definition of x1 and x2 possible hide definitions of these keys inside f or in case x1 = x2 the middle update is void. You easily get into a lot of aliasing problems while proving thus a statement. However, the following theorem holds:

   |- !f x y. FEVERY P (f |+ (x,y)) /\ P (x,y) ==> FEVERY P (f |+ (x,y))

Given a directed consequence conversion c that instantiates this theorem, DEPTH_CONSEQ_CONV can be used to apply it repeatedly and at substructures as well:

  DEPTH_CONSEQ_CONV c CONSEQ_CONV_STRENGTHEN_direction
     ``!y2. FEVERY P (f |+ (x1, y1) |+ (x2,y2)) /\ Q z`` =


  |- (!y2. FEVERY P f /\ P (x1, y1) /\ P (x2,y2) /\ Q z) ==>
     (!y2. FEVERY P (f |+ (x1, y1) |+ (x2,y2)) /\ Q z)

SEEALSO

DEPTH_CONV, ONCE_DEPTH_CONSEQ_CONV, NUM_DEPTH_CONSEQ_CONV, DEPTH_STRENGTHEN_CONSEQ_CONV, REDEPTH_CONSEQ_CONV