Optional arguments are defined as follows:
[c val] . This means that c is an optional argument with
default value val. val can be a scalar: e.g. [c 10],
an array: e.g. [c (4)/1,2,3,4/] or a chain: e.g. [c pipo]
{b xx}. This means that b is an optional argument.
If not found, one looks for xx in current existing Scialb variables.