Library /sys$common/syshlp/helplib.hlb  —  MACRO  /ALPHA  Directives  .UNDEFINE REG, Description
    The register symbol that you specify as the argument to the
    .UNDEFINED_REG directive is no longer a register symbol. Starting
    with the statement that follows the .UNDEFINE_REG directive, you
    can use the symbol as a MACRO-64 identifier.
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