model.matrix.default chokes on backquote (PR#7202)

ggrothendieck@myway.com writes:

The following gives an error:

	> `a(b)` <- 1:4
	> `c(d)` <- (1:4)^2
	> lm(`a(b)` ~ `c(d)`)

	Error in model.matrix.default(mt, mf, contrasts) : 
        	model frame and formula mismatch in model.matrix()

To fix it replace this line in model.matrix.default:

	reorder <- match(attr(t, "variables")[-1], names(data))

with these two lines:

	strip.backquote <- function(x) gsub("^`(.*)`", "\\1", x)
	reorder <- match(strip.backquote(attr(t, "variables"))[-1], 
			strip.backquote(names(data)))

Hmm.. Yes, there's a bug (and it's likely not the only one we have
relating to odd variable names in model formulas), but I suspect that
the fix is wrong. 

The backquotes are not part of the variable names, but get added by
deparsing -- sometimes! Other times they do not: Try for instance
as.character(quote(`a(b)`)). (Which is as it should be. Other pieces
of logic relating to nonsyntactical names represent some rather
awkward compromises.)

When backquotes have found their way into names(data) or the
"variables" attribute, I would rather suspect that they were created
by the wrong tool and fix that, not cure the symptom by stripping them
off at a later stage.

O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907