Forcing a natural accidental at the start of a bar/measure

[Padmavyuha]

Hello there

I've just begun using Lilypond (v2.12.2-1) on my mac (and wading  
through the manual!).

The first problem I've encountered is with accidentals. In an SATB 4- 
staff score, I've got the following two bars in a staff (in 4/4, scale  
set to D major):

ef2. ef4
e2 d8 cs b a

but no matter what I do with extraNatural, accidentalStyle etc.  
settings, I can't get the E natural in the 2nd bar to show a natural  
accidental, which it really needs to do.

How can I force this accidental?

Any help appreciated!

- Padmavyuha