finding missing lines...

Take this as an example:

a=data.frame(col1=c(1,2,3,4,5), col2=c

("my","beloved","daughter","son","wife"))

b=data.frame(col1=c(1,2,4), 

col2=c("my","beloved","son"))

a

col1     col2
1    1       my
2    
2  beloved
3    3 daughter
4    4      son
5    5     wife

b

col1    col2
1    1      my
2    2 beloved
3    4     son

As you can 
see in b is equal to a with exception of lines of a with col1= 3 and 
col1=5 that are missing.

How can I obtain a third dataframe made of 
the missing lines only, e.g.

c

col1     col2
1    3 daughter
2    
5     wife

Ciao
Vittorio