FUNCTION: IsBorder - test whether the argument is a border of a partition

CALLING SEQUENCE:

IsBorder(expr)

TYP[IsBorder](expr)

PARAMETERS:

expr = any expression

SYNOPSIS:

• The IsBorder function tests whether expr is a border list, 0 denoting an horizontal segment of the border, 1 a vertical one. Extremity segments shall not be included (i.e. 1 on the left top, 0 on the right bottom).

• When called with a second argument 'alphabet'=m, where m is list of two elements, IsBorder returns the same, m[1] denoting an horizontal segment of the border, m[2] a vertical one.

• Whenever there is a conflict between the function name IsBorder and another name used in the same session, use the long form TYP['IsBorder'].

EXAMPLES:

> with(TYP):
> IsBorder([0,1,0]);
 
                                    false
 
> IsBorder([0,1,0,0,1,1]);
 
                                     true
 
> IsBorder([a,b,a,a,b,b], 'alphabet'=[a,b]);
 
                                     true