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3.1.3 Standard Parameter Names
In the descriptions of the built-in functions, several generic
names are used for parameters, to indicate something about the
type and use of that parameter, e.g. valid range. To avoid
repeating the same information for the majority of the functions,
some common “rules” for the standard parameter names are stated
here. These rules implicitly apply for the rest of this chapter.

Note that the following list does not try to classify any general
REXX “datatypes”, but provides a binding between the sub-datatypes
of strings and the methodology used when naming parameters.

·    Length is a non-negative whole number within the internal
  precision of the built-in functions. Whether it denotes a length
  in characters or in words, depends on the context.

·    String can be any normal character string, including the
  nullstring. There are no further requirements for this parameter.
  Sometimes a string is called a “packed string” to explicitly show
  that it usually contains more than the normal printable

·    Option is used in some of the functions to choose a
  particular action, e.g. in DATE() to set the format in which the
  date is returned. Everything except the first character will be
  ignored, and case does not matter. note that the string should
  consequently not have any leading space.

·    Start is a positive whole number, and denotes a start
  position in e.g. a string. Whether it refers to characters or
  words depends on the context. The first position is always
  numbered 1, unless explicitly stated otherwise in the
  documentation.  Note that when return values denotes positions,
  the number 0 is generally used to denote a nonexistent position.

·    Padchar must be a string, exactly one character long.  That
  character is used for padding.

·    Streamid is a string that identifies a REXX stream. The
  actual contents and format of such a string is implementation

·    Number is any valid REXX number, and will be normalized
  according to the settings of NUMERIC before it is used by the

If you see one of these names having a number appended, that is
only to separate several parameters of the same type, e.g.
string1, string2 etc. They still follow the rules listed above.
There are several parameters in the built-in functions that do not
easily fall into the categories above. These are given other
names, and their type and functionality will be described together
with the functions in which they occur.