Mutability of loop variables

[vsajip]

Is it possible to write a loop such that the loop variable can be assigned to? 
The following example:
    
    
    import strutils
    
    let s = "a \n b\n c \n"
    
    for line in s.splitLines:
        line = line.strip
        # TODO further procession of line
    

fails to compile, with a `Error: 'line' cannot be assigned to`. Can this be 
avoided, or does another variable have to be introduced to get around it?