conditional \when-property

[Kieren MacMillan]

Hello all,

This \when-property function (whence I found it, I can't remember) clearly 
doesn't work as expected:

\version "2.17"

#(define-markup-command (when-property layout props symbol markp) (symbol? 
markup?)
  (if (chain-assoc-get symbol props)
      (interpret-markup layout props markp)
      (ly:make-stencil '()  '(1 . -1) '(1 . -1))))

\paper {
  scoreTitleMarkup = \markup \column {
    \when-property #'header:foo { \fromproperty #'header:foo }
    \when-property #'header:bar { \fromproperty #'header:bar }
    \when-property #'header:foo { \fromproperty #'header:foo }
    \when-property #'header:bar { \fromproperty #'header:bar }
    \when-property #'header:foo { \fromproperty #'header:foo }
    \when-property #'header:bar { \fromproperty #'header:bar }
    \when-property #'header:foo { \fromproperty #'header:foo }
    \when-property #'header:bar { \fromproperty #'header:bar }
    \fromproperty #'header:title
  }
}

\header {
  title = TEST
}

\score {
  \relative c' { c1 }
}

How can it be made truly conditional (so that the titling line/item takes up no 
vertical space if the property is null)?

Thanks,
Kieren.