sml-mode indentation for structures

[Helmut Eller]

I'm using sml-mode version 6.5 from ELPA, but the indentation of
structures drives me nuts.  I would like to use a style like this:

structure Foo =
  struct
    fun foo x = x
  end

but by default sml-mode indents it like this:

structure Foo =
struct
fun foo x = x
end

Can somebody tell me how sml-mode indentation rules can be configured?

Helmut