          env
               This variable is maintained by Tcl as  an  array  whose
               elements are the environment variables for the process.
               Reading  an  element  will  return  the  value  of  the
               corresponding environment variable.  Setting an element
               of the array will modify the corresponding  environment
               variable  or  create  a  new  one if it doesn't already
               exist.  Unsetting an element of  env  will  remove  the
               corresponding environment variable.  Changes to the env
               array will affect the environment passed to children by
               commands  like  exec.  If the entire env array is unset
               then Tcl will stop monitoring env accesses and will not
               update environment variables.
