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A portable, multi-platform, command-line driven graphing utility

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Commit [2cf853] Maximize Restore History

Guarantee that comma operator is a sequence point in serial evaluation

This commit adds an operation SERIAL_COMMA to the evaluation stack
corresponding to the position of a comma in an expression such as
A = ( <exp1> , <exp2> )
The intent is that any error flags from evaluation of <exp1> should not
persist across evaluation of <exp2>. In particular, if <exp2> is
well-defined but <exp1> is not then A should receive the value of <exp2>
rather than being set to <undefined>.

gnuplot> show at A = (B,C)

pushc "A"
push B
pop
serial comma
push C
assign

This change allows serial evaluation in a "using" clause to reference a missing
or NaN value as part of <exp1> without affecting the value return for <exp2>.
plot FOO using 1 : (TEMP=$3, $2)
The point at [ $1, $2 ] should be plotted even if column 3 is missing, NaN,
or undefined. Further use of the value in TEMP later in the plot command
may be problematic, however.

Bug #2787

Authored by: Ethan A Merritt 2025-04-15