Mine

<elema attr1=a   attr2=b   attr3=c />
<elemb attr1=aa  attr2=bb  attr3=cc /> 
<elemc attr1=aaa attr2=bbb attr3=ccc /> 
   ->

<elema attr1=d   attr2=b   attr3=c  />
<elemb attr1=d   attr2=e   attr3=cc /> 
<elemc attr1=d   attr2=bbb attr3=f  />  



./foo -n -t elema,elemb,elemc -a attr1 -v d\
         -st elemb            -a attr2 -v e\
         -tt elemc            -a attr3 -v f