[R] Symbolic substitution of expressions into expressions

[John Maindonald]

In the following

titletxt <- substitute(paste(tx, tilde(y) == sqrt(y)),
  list(tx="Replace y by "))

I'd like to be able to replace e.g. sqrt(y) by a symbol or
symbolic expression that has the effect of inserting sqrt(y)
into the place that it occupies.  This would allow me to put an
arbitrary expression into that position.  [One can use text() or
mtext() etc. to place titletxt on a graph.]

Any clues would be most welcome.
John Maindonald.
John Maindonald               email : john.maindonald at anu.edu.au        
Statistical Consulting Unit,  phone : (6125)3998        
c/o CMA, SMS,                 fax   : (6125)5549  
John Dedman Mathematical Sciences Building
Australian National University
Canberra ACT 0200
Australia
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