Converting compositions to strings

[Clément Pit-Claudel]

Hi emacs-devel,

Many compositions can be represented as strings: for example, 
prettify-symbols-mode composes "lambda" into "λ", and nameless prettifies the 
name of the current package as ":" or "::".

How can I reconstruct these strings from the value of the 'composition' text 
property?  Small experiments suggest that for single-character compositions the 
property has the shape (_ _ [?λ]) and for multi-character compositions (_ _ [?x 
_ ?y _ …]), but I also found instances of the shape ((_ ?x _ _)), and ((_ . 
?x)) (I think).

Is there a recipe that I can use to construct a string representation of a 
composition? (I realize that many compositions can't adequately be represented 
as plain strings; I'm only interested in those that can).

Thanks!
Clément.