IF  	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 ~=.
  	For example:
  
  	       IF I == J
  	          A(I,J) = 2;
  	       ELSEIF ABS(I-J) == 1
  	          A(I,J) = -1;
  	       ELSE
  	          A(I,J) = 0;
  	       END