`convert/f`(a,x,y,...) = convert(a,f,x,y,...)
matrix, array,
vector et listlist) (cf. i.5.2)
> convert([a,b,c],`+`);
	                a+b+c
> convert([[a],[b],[c]],array);        
                        [ a ]
                        [ b ]
                        [ c ]