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OS_EvaluateExpression is documented as applying GSTrans to any strings
within the expression, but did instead GSTrans the entire expression.

This would result in a sequence such as:

    *Set Alias$@RunType_FD1 "Basic -quit ""%0"" %*1"
    *If "<Alias$@RunType_FD1>"="" Then Set Alias$@RunType_FD1 @RunType_FFB %*0

raising a "Unknown operand" error as the quote in the variable
were interpreted as expression syntax. See
https://www.riscosopen.org/forum/forums/5/topics/16127

Unfortunately skipping the initial GSTrans breaks common code such as:

    If "<StrED_cfg$Dir>" = "" AND <Ctrl$Pressed> = 0 Then ...
    If "<Zap$OSVsn>" <> "" Then SetEval ZapFonts$OSVsn <Zap$OSVsn>

Fix by applying a GSTrans transformation over unquoted parts of the
expression, then applying GSTrans over the quoted strings.

Version 6.51. Tagged as 'Kernel-6_51'