TCC > Variables & Functions > Functions:
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@SUBST[n, string1, string2]: Substitutes string1 starting at position n in string2.
Numeric input may be entered in either decimal format (a sequence of 0-9 digits) or in hexadecimal format ("0x" followed by a sequence of 0-F hex digits).
Help URL: http://jpsoft.com/help/index.htm?f_subst.htm