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2.4.5 The DROP Instruction
     DROP symbol [ symbol ... ] ;

The DROP instruction makes the named variables uninitialized, i.e.
the same state that they had at the startup of the program. The
list of variable names are processed strictly from left to right
and dropped in that order.  Consequently, if one of the variables
to be dropped is used in a tail of another, then the order might
be significant. E.g. the following two DROP instructions are not

     bar = ‘a’
     drop bar  /* drops ‘BAR’ and ‘FOO.BAR’ */
     bar = ‘a’
     drop bar  /* drops ‘FOO.a’ and ‘BAR’

The variable terms can be either a variable symbol or a symbol
enclosed in parentheses. The former form is first tail-
substituted, and then taken as the literal name of the symbol to
be dropped. The result names the variable to drop. In the latter
form, the value of the variable symbol inside the parentheses is
retrieved and taken as a space separated list of symbols. Each of
these symbols is tail-substituted (if relevant); and the result is
taken as the literal name of a variable to be dropped. However,
this process is not recursive, so that the list of names referred
to indirectly can not itself contain parentheses. Note that the
second form was introduced in TRL2, mainly in order to make
INTERPRET unnecessary.

In general, things contained in parentheses can be any valid REXX
expression, but this does not apply to the DROP, PARSE, and
PROCEDURE instructions.

Example: Dropping compound variables

Note a potential problem for compound variables: when a stem
variable is set, it will not set a default value, rather it will
assign “all possible variables” in that stem collection at once.
So dropping a compound variable in a stem collection for which the
stem variable has been set, will set that compound variable to the
original uninitialized value; not the value of the stem variable.
See section Assign for further notes on assignments.  To
illustrate consider the code:

      foo. = ‘default’
      drop baz bar
      say foo.baz /* says ‘FOO.BAR default’ */

In this example, the SAY instruction writes out the value of the
two compound variables FOO.BAR and FOO.BAZ. When performing tail-
substitution for these, the interpreter finds that both BAR and
BAZ are uninitialized. Further, FOO.BAR has also been made
uninitialized, while FOO.BAZ has the value assigned to it in the
assignment to the stem variable.

Example: Tail-substitution in DROP

For instance, suppose that the variable FOO has the value bar.
After being dropped, FOO will have its uninitialized value, which
is the same as its name: FOO. If the variable to be dropped is a
stem variable, then both the stem variable and all compound
variables of that stem become uninitialized.

      bar = 123
      drop   /* drops ‘FOO.123’ */

Technically, it should be noted that some operations involving
dropping of compound variables can be very space consuming. Even
though the standard does not operate with the term “default value”
for the value assigned to a stem variable, that is the way in
which it is most likely to be implemented. When a stem is assigned
a value, and some of its compound variables are dropped
afterwards, then the interpreter must use memory to store
references to the variables dropped. This might seem
counterintuitive at first, since dropping ought to release memory,
not allocate more.

There is a parallel between DROP and PROCEDURE EXPOSE.  However,
there is one important difference, although PROCEDURE EXPOSE will
expose the name of a variable enclosed in parentheses before
starting to expose the symbols that variable refers to, this is
not so for DROP. If DROP had mimicked the behavior of PROCEDURE
EXPOSE in this matter, then the whole purpose of indirect
specifying of variables in DROP would have been defeated.

Dropping a variable which does not have a value is not an error.
There is no upper limit on the number of variables that can be
dropped in one DROP clause, other than restrictions on the clause
length.  If an exposed variable is dropped, the variable in the
caller is dropped, but the variable remains exposed. If it
reassigned a value, the value is assigned to a variable in the
caller routine.