fix issue fricas#142

The problem is independent of ")set stream calc 1".
It just happened to be hidden.

Before this commit the code below resulted in the values 0, 5, 3
for the last 3 commands.

ZZ ==> Integer
SULS ==> SparseUnivariateLaurentSeries(ZZ, 'q, 0)
Rec ==> Record(k: ZZ, c: ZZ)
S ==> Stream Rec
st := cons([0,3]$Rec, cons([1,5]$Rec, cons([2,7]$Rec,empty()$S)))$S
s := series(st)$SULS;
i x ==> x pretend InnerSparseUnivariatePowerSeries ZZ
explicitEntries?(getStream i s)
getRef i s
coefficient(s,0)
coefficient(s,1)
coefficient(s,0)

src/algebra/sups.spad

@@ -215,7 +215,7 @@ InnerSparseUnivariatePowerSeries(Coef) : Exports == Implementation where
 
     series(x : ST) ==
       empty? x => 0
-      n := getExpon frst x; refer := ref(n :: COM)
+      n := getExpon frst x; refer := ref((n-1) :: COM)
       makeSeries(refer, iSeries(x, refer))
 
 --% values

src/input/bugs2023.input

@@ -110,4 +110,8 @@ testcase "polynomial roots"
 
 testEquals("rootSimp((4*a^6)^(1/4))", "a*sqrt(2*a)")
 
+testcase "initial coefficients of sparse univariate power series"
+
+testEquals("st:=[[n,n+42]@Record(k:INT,c:INT) for n in 0..];s:=series(st)$InnerSparseUnivariatePowerSeries(INT);coefficient(s,0)", "42")
+
 statistics()