one of the documented procedures in this installation of the ACS
export_var   var_name   { default "" }
What it does:
Returns a variable's value if it exists. This can be used to protect against undefined variables.
Defined in: /web/philip/tcl/utils-extra.tcl

Source code:

    # export_var protects against undefined variables
    # Returns the value of the variable if it exists in the caller's environment. 
    # Otherwise, returns default.

    if [eval uplevel {info exists $var_name}] {
	upvar $var_name value
	return "$value"
    } else {
	return "$default"