1 + f // N
1. + f[1.]
I don't want the argument of
f evaluated by N; I want to get
1. + f
In general, there is a large algebraic expression containing some subexpressions of type
N should not be applied inside
N should be applied only outside.
N at a certain level only is not OK, since the tree structure of expression is not know in advance.
One possible trick is
1 + f /. x_f :> 1. x + 0.
It would be good to know other options.