tag and default

[Gianmaria Lari]

This code ...

\version "2.19.81"

var = { 

  \tag #'mya {a}

  \tag #'myb {b}

}

{ \keepWithTag #'mya \var }

{ \keepWithTag #'myb \var }

{ \var}

creates three scores:

1) a

2) b

3) a b

In the third case, where no \keepWithTag is specified, lilypond returns all elements (a and b).

Is there any way to make lilypond returning only one element (a sort of default element) and not all the elements IF I don't specify any \keepWithTag? 

Thank you, g.