TieColumn syntax

[Flaming Hakama by Elaine]

Hi, 

I wanted to set a laissez vibrer tie upwards, and found that there is no equivalent to \slurUp, \tieUp, etc.

I was able to do it, thanks to http://lsr.di.unimi.it/LSR/Snippet?id=211

In my use case, the following did the trick:

\version "2.19.81" 

laissezVibrerUp = \once \override LaissezVibrerTieColumn.tie-configuration = #`((2.5 . ,UP))

\relative c' { r2 f8 a b d \laissezVibrer }
\relative c' {  r2 f8 a b \laissezVibrerUp d \laissezVibrer }

However, I am curious about the syntax of the tie-configuration.

I get that it is a list of pairs, and in my case the list only has one element.  

What is the meaning of the leading comma for the direction? 

Also, why does the IR only discuss the values 1, -1 and 0, when there are also descriptive names for the direction values?  I'm guessing that the IR is a lower level interface, and that, for example,  ",UP" ends up being a convenience for the value 1.

Is there somewhere else in the docs where this is described?

http://lilypond.org/doc/v2.19/Documentation/internals/tie_002dcolumn_002dinterface

tie-configuration (list)

List of (position . dir) pairs, indicating the desired tie configuration, where position is the offset from the center of the staff in staff space and dir indicates the direction of the tie (1=>up, -1=>down, 0=>center). A non-pair entry in the list causes the corresponding tie to be formatted automatically.

Thanks, 

Elaine Alt

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