-2956 You may not set display field field-tag to a value because
none of its associated database columns belong to the same table as
table, nor is it a variable display-only field.
You cannot assign values in a LET statement across tables. All columns
that are associated to the field-tag in a LET statement must belong to
tables that are named in the opening command of the control block, or
it must be a display-only field.
