execGRASS() for r.mapcalc?

I'm used to do:
 > cmd = "r.mapcalc 'delme=100'"
 > system(cmd)

and works, but how could I do the equivalent using execGRASS() ?
I've tried:

 > doGRASS(cmd)
Error in parseGRASS(cmd) : r.mapcalc 'delme=100' not parsed

 > execGRASS(cmd)
Error in parseGRASS(cmd) : r.mapcalc 'delme=100' not parsed

I can always use paste() to make any cmd string, but seems to me that 
execGRASS() would be cleaner.

Thanks

Agus