External factors

The external factors are described by means of the syntax:

<external factor spec> ::= (external-factor <external factor name>
                                            [ <comment> ]
                                            [:unit <symbol>] 
                                            [:range (<min> <max>)]
                                            :baseline-value <number>
                           )
<external factor name> ::= <symbol>
<min>                  ::= <number>
<max>                  ::= <number>
<comment>              ::= <string>