:- dynamic wd/2, tag/2, tag/3.
:- discontiguous(wd/2, tag/2, tag/3).

%%% DATA REPRESENTATION
wd(1,'Should').   tag(1,md).   tag(md,md,1). 
wd(2, I).         tag(2,pn).   tag(pn,pn,2). 
wd(3, apologize). tag(3,vbp).  tag(vbp,vb,3). 
wd(4, for).       tag(4,in).   tag(in,in,4). 
wd(5, the).       tag(5,dt).   tag(dt,dt,5). 
wd(6, wait).      tag(6,vb).   tag(vb,nn,6). 
wd(7, '?').       tag(7,'.').  tag(.,.,7). 

%%% TEMPLATES                 %Condition the transformation rule on...
pos:A>B <- pos:C@[-1,-2].     %  current pos and any of the two previous pos
pos:A>B <- wd:C@[-1].         %  current pos and previous word

%%% LEARNED RULES             %When tagging new data from a baseline...
pos:vbp>vb <- pos:md@[-1,-2]. %  retag vbp to vb when any of the two previous pos is "md"
pos:vb>nn  <- wd:the@[-1].    %  retag vb to nn when the previous word is "the"