Allow indicated parameters to vary. (See also freeze)
where <param range> ::= [<modelName>:]<param#>| <param#>- <param#>.
For response parameters (see gain command):
where <param range> ::= [<source number>:]<param#>| <param#>- <param#>.
The indicated parameter, or range of parameters, will be marked as variable
by the fitting commands and treated as a fitting parameter in subsequent
fits. By default, the range will be the last range input by either a
freeze or thaw command. See the freeze examples for an
example of the use of the thaw command.