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Merge mappings with equal SQL table and subject map

convert mappings which just project different columns from one table into one TripleMap, assuming this is safe (which I guess amounts to checking if the subjectMaps are equal).

So the two mappings (map:m-1, map:m-2) below (they only differ on y1/y2), should be trandformed into map:supermap, also below, which uses tt:table instead of R2RMLView, and contains multiple predicateObjectMap-s. The benefit is at least less duplication of the subjectMap.

map:m-1
   a rr:TriplesMap ;
   rr:logicalTable [
       a rr:R2RMLView ;
       rr:sqlQuery """
   SELECT
       x, y1 from tableA"""
   ] ;
   rr:predicateObjectMap [
       a rr:PredicateObjectMap ;
       rr:objectMap [
           a rr:ObjectMap, rr:TermMap ;
           rr:column "y1" ;
           rr:termType rr:Literal
       ] ;
       rr:predicate npdv:myY1
   ] ;
   rr:subjectMap [
       a rr:SubjectMap, rr:TermMap ;
       rr:template "http://sws.ifi.uio.no/data/{x}" ;
       rr:termType rr:IRI
   ] .

map:m-2
   a rr:TriplesMap ;
   rr:logicalTable [
       a rr:R2RMLView ;
       rr:sqlQuery """
   SELECT
       x, y2 from tableA"""
   ] ;
   rr:predicateObjectMap [
       a rr:PredicateObjectMap ;
       rr:objectMap [
           a rr:ObjectMap, rr:TermMap ;
           rr:column "y2" ;
           rr:termType rr:Literal
       ] ;
       rr:predicate npdv:myY2
   ] ;
   rr:subjectMap [
       a rr:SubjectMap, rr:TermMap ;
       rr:template "http://sws.ifi.uio.no/data/{x}" ;
       rr:termType rr:IRI
   ] .
map:supermap
   a rr:TriplesMap ;
   rr:logicalTable [ rr:tableName "tableA" ]
   rr:predicateObjectMap [
       a rr:PredicateObjectMap ;
       rr:objectMap [
           a rr:ObjectMap, rr:TermMap ;
           rr:column "y1" ;
           rr:termType rr:Literal
       ] ;
       rr:predicate npdv:myY1
   ] ;
   rr:predicateObjectMap [
       a rr:PredicateObjectMap ;
       rr:objectMap [
           a rr:ObjectMap, rr:TermMap ;
           rr:column "y2" ;
           rr:termType rr:Literal
       ] ;
       rr:predicate npdv:myY2
   ] ;
   rr:subjectMap [
       a rr:SubjectMap, rr:TermMap ;
       rr:template "http://sws.ifi.uio.no/data/{x}" ;
       rr:termType rr:IRI
   ] .

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