dependency difficulties

Dear List,

I'm new in XSLT, and i find some cases hard. I have two questions:

1,
Is it able to add a <xsl:template match="...">...</xsl:template> section
to an exsisting, working xslt without changing the orginal behaviour of
the exsiting xslt? (like adding an event handler to an object will not
change the call-order of other callbacks.) 
(There is a point in an xslt development where the clear-seeing of
dependencies will be very difficult, how can I maintain the interaction
dificulties?)

2,
How can I cut a subtree?

e.g:

a:
  -A
   |-B
   |-C
   | |-I
   |   |-J
   |   |-K
   |-D
     |-E

b:
    -A
     |-C
     | |-I
     |   |-K
     |-D
       |-E

a-b:

    -A
     |-B
     |-C
       |-I
         |-J


Thanks,
Gza


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