Frequency selection expression consists of two specifications separated by colon (':') in the form:
SPWSPEC[:CHANSPEC]
where SPW is the spectral window specification and CHANSPEC is the
optional frequency specification for selection within the selected
spectral windows. When channel specification is omitted, all channels
of the selected SPW are selected.
SPWSPEC and CHANSPEC allow use of semi-colon separated lists. As a result, list of SPWSPEC can be given in two ways: (1) a semi-colon separated list, (2) coma separated list. This allows simple selections to remain simple and more complex selections consistent with the syntax for CHANSPEC list.