[un]lock {<variable> [<variable> ]...|all}

The "lock" command prevents a defined variable's value from being altered by any of the normal processes that change or clear variable values.  The "lock" command can not pre-lock undefined variables, but additional locks can appear automatically based on equation constraints.  The "unlock" command removes locks, allowing the variables to change normally.

The "all" keyword allows locking or unlocking all defined variables at once.

Note:  The "lock" command is a non-destructive alternative to the "constant" command since a "lock" can always be undone with an "unlock" command.

e.g.   y=mx+b       Enter a simple equation with four unknowns.
  m=3.7 Define an unknown.
  b=2 Define another unknown.
  x=17 Define "x". "y" automatically gets evaluated.
  lock m b Lock the values of some "fixed" variables.
  x=23 Re-define "x". Only "y" clears & re-evaluates!
  unlock all Remove all locks.
See also:  clear, constant, equation, list