global staff-staff spacing fails in 2.13.46

[James Worlton]

Hello,

The following code compiles,  but the spacing override in the layout  
block fails. The layout block code was copied and pasted into my test  
directly from the NR 4.4.1 "Within-system spacing properties".  
According to the docs, this is supposed to apply globally to all  
staves within systems.

\version "2.13.46"
\score {
    <<
        \new Staff { c'1 }
        \new Staff { c'1 }
    >>
    \layout {
        \context {
            \Staff
            \override VerticalAxisGroup #'staff-staff-spacing  
#'basic-distance = #10
        }
    }
}

This fails with:

Parsing...
Interpreting music...
warning: type check for `staff-staff-spacing' failed; value `((basic- 
distance . 10) . #<primitive-procedure ly:axis-group-interface::calc- 
staff-staff-spacing>)' must be of type `list'
warning: type check for `staff-staff-spacing' failed; value `((basic- 
distance . 10) . #<primitive-procedure ly:axis-group-interface::calc- 
staff-staff-spacing>)' must be of type `list'
Preprocessing graphical objects...
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
Layout output to `spacetest.ps'...
Converting to `./spacetest.pdf'...
success: Compilation successfully completed

Putting the override in a Staff's \with block gives the same error.  
However, writing the command as a list:
\override VerticalAxisGroup #'staff-staff-spacing = #'((basic- 
distance . 50))
compiles fine and does what's expected.

Is this a bug/regression, or has the ability to define a single  
variable, as in my example, gone away and the docs just not been  
updated to reflect that?

James Worlton