x,y,ib,region, nx, ny .
x is horizontal and y is vertical.

On pdOmega it is equal to the boundary identification number.
The usual if... then ... else statement can be used with an important restriction on the logical expression which must return a scalar value:
if( logical expression) then
{
statement;
....;
statement;
}
else
{
.....
};
The logical expression controls the if by its return being 0 or >0,
it is evaluated only once (i.e. with x, y being the coordinates of the
first vertex, if there are functions inside the logical expression).
Auxiliary variables can be used.
In order to minimize the memory the symbol := tells the compiler not
to allocate a data array to this variable. Thus v=sin(a*pi*x);
generates an array for v but no array is assigned to a in the
statement a:=2 .