the value
of the string contained in <I>objPtr</I> and scans it, copying
characters and performing the chosen substitutions as it goes to an
output value which is returned as the result of the function.  In the
event of an error occurring during the execution of a command or
variable substitution, the function returns NULL and an error message
is left in <I>interp</I>'s result.
<P>
Three kinds of substitutions are supported.  When the
<B>TCL_SUBST_BACKSLASHES</B> bit is set in <I>flags</I>, sequences that
l