Numeric value required condition in NUMERIC component

Repeatable REXX development requires predictable numeric conversion behavior across implementations.

The REXX standard does not definitively specify whether an empty string used in a numeric context should be treated as zero or should trigger a numeric value required condition. This ambiguity affects arithmetic expressions and comparisons and limits repeatability in cross-implementation builds. Behavior is version and implementation sensitive for IBM z/OS REXX, Regina REXX and ooRexx, with differences in NUMERIC DIGITS defaults and condition handling.

What condition class is raised, if any, when an empty string participates in arithmetic under classic REXX semantics? Is empty string coerced to zero by default in IBM z/OS REXX, Regina REXX and ooRexx? What information is available to the caller to detect divergent conversion?