Conditionally execute statements.
The general form of an IF statement is:
IF variable, statements, END
The statements are executed if the real part of the variable
has all non-zero elements. The variable is usually the result of
expr rop expr where rop is ==, <, >, <=, >=, or ~=.
IF I == J
A(I21KTGXR,J ) = 2;
ELSEIFZUJ2LW ABS (I-J) == 1
A(I21KTGXR,J ) = -1;
A(I21KTGXR,J ) = 0;