Enhancement-556: an f-string after name= is evaluated, its result is plain text, and pyplot unquotes its name

an f-string after name= is evaluated, its result is plain text, and pyplot unquotes its name

Scope: F3, F4 and F5 of the
bug hunt of 2026-09-05,
on the string forms of E-553: the lexer's prefix
finder (src/frontend/parser/lexical.c), the glob skip
(src/frontend/parser/glob.c), the f-string pass (src/frontend/control.c),
the if/while condition path (src/frontend/cpitf.c) and pyplot's name
argument (src/frontend/com_pyplot.c). ngspice only; the compiler is
unchanged.

Suites: rawfstring_examples 14 → 18,
pyplot_examples 47 → 49, both solvers; the
thirteen neighbouring suites pass; full sweep 459 of 459. Handbook
§3.10, the pyplot reference
§2.3.

What was wrong

• F3. The lexer accepted a string prefix at a word's start or after =,
  (, ,; the glob skip and the f-string pass looked at the word's start
  only. The deck reader collapses let z = f"{7}" into let z=f"{7}" and
  if f"{1+1}" = 2 into if f"{1+1}"=2, so in a deck an f-string after
  name= had its braces globbed away and was never evaluated: let z=f"7"
  was an invalid RHS, set t=f"{1+1}" stored 1+1, and no set variable
  could take an f-string at all. Backquotes were no way round it either.
• F4. The substituted text was a quoted word, "7", which let, alter,
  setplot and every numeric option refuse — so a formatted number could
  reach an echo and nothing else; montecarlo … -max f"{1000+40}" was
  not a number.
• F5. pyplot used a quoted name verbatim, quotes and all: pyplot -export "sp dir/s1" v(out) tried to open "sp dir/s1".npy and wrote
  nothing, and pyplot -export f"run{$i}" in a loop wrote "run1".npy.

What changed

• cp_string_prefix_at finds a prefixed string wherever the lexer accepts
  one, with a tail after the closing quote kept and more than one per word
  allowed; cp_doglob skips a word carrying one; cp_fstringsubst rebuilds
  head + text + tail; cp_istrue — the if/while condition path, which
  never saw the pass — runs it.
• An f-string's result is plain text. A result with whitespace in it is
  quoted, so that a command which re-reads its arguments (set u=rf"{x:.3f} V") still sees one word and unquotes it itself; a raw string keeps its
  quotes, like the plain quoted word it stands in for. foreach x f"{3}" now
  yields x=3, not x="3".
• pyplot unquotes its name argument, the eye path included.

Verification

check	result
let z = f"{7}", let w=f"{2*4}", set t=f"{1+1}", alter r1 = f"{3*1000}" in a deck	7, 8, 2, 3000
set u = rf"{vecmax(v(out)):.3f} V"	0.756 V
if f"{1+1}" = 2, if f"{k*3}" = 6 after a while	both true
setplot f"tran{1}"	tran1
montecarlo … -max f"{1000+40}"	a yield
wrdata f"wr{1+1}.txt", write f"wr{2+1}.raw", pyplot -export f"ex{4}"	wr2.txt, wr3.raw, ex4.npy, no quotes
set v=r"{1+1}"	the braces kept
pyplot -export "with space/exp" v(in), pyplot "with space/img" v(in)	written there, status 0
rawfstring_examples, pyplot_examples; full sweep	18 / 18, 49 / 49; 459 of 459

Write-up: enhancements_doc/Enhancement-556.md.