Consider the following two disjunctions:

The definition sentence consists of an IF..THEN..ELSE..ENDIF sentence. The syntax for a single disjunction is:

disjunction_name IS
IF term_condition THEN
        constraint_name_1;
        …
        constraint_name_i;
        …
        constraint_name_n;
[ELSE] | [ELSIF term_condition THEN]
        constraint_name_n+1;
        …
        constraint_name_r;
        …
        constraint_name_z;
ENDIF;

For the example presented we have two disjunctions that can be declared and defined as follows:

$ONECHO > "%lm.info%"

Disjunction D1, D2;

D1 IS
        IF Y('1') THEN
                CONSTRAINT1;
                EQUATION1;
        ELSIF Y('2') THEN
                CONSTRAINT2;
                EQUATION2;
        ENDIF;

D2 IS
        IF Y('3') THEN
                CONSTRAINT3;
        ELSE
                CONSTRAINT4;
        ENDIF;

$OFFECHO

Declaration and definition of CONSTRAINT1, CONSTRAINT2, CONSTRAINT3, CONSTRAINT4, EQUATION1 and EQUATION2 must be made in GAMS section of the input file.