Concatenating sas macro variables

I always forget how to do it.

%let nagasuchi=cases;
data new;
set library.old&nagasuchi;

This would be read as
data new;
set library.oldcases;


If the macro variable is a prefix

data new;
set library.&nagasuchi.old;
/* note the period*/

This would be read as
data new;
set library.casesold;

If the character following a macro variable is a period, then you need to use two periods.
set in&nagasuchi..select;

After resolution, SAS would read this as SET incases.select;


More on this here.