xmodmap is malfunctioning under Debian as of 2010-09-04. If you don't use Gnome, but want to install StumpWM under Debian on MacBook Pro, you will get errors of the following type when trying to manipulate modifiers like Control etc.:

X Error of failed request: BadValue (integer parameter out of range for operation)
  Major opcode of failed request: 118 (X_SetModifierMapping)
  Value in failed request: 0x17
  Serial number of failed request: 16
  Current serial number in output stream: 16

To use StumpWM, the best way is to put left and right Control keys next to the space bar. To do this, as per xmodmap failure BadValue error, 118 (X_SetModifierMapping) from the Ubuntu bug database, do:

<LALT> = 64;
<LCTL> = 37;
<SPCE> = 65;
<RCTL> = 105;
<RALT> = 108;
// Microsoft keyboard extra keys
<LWIN> = 133;
<RWIN> = 134;
with
<LALT> = 108;
<LCTL> = 133;
<SPCE> = 65;
<RCTL> = 134;
<RALT> = 64;
// Microsoft keyboard extra keys
<LWIN> = 37;
<RWIN> = 105;

.