Index: lib/numeric/modArith.cpp
==================================================================
--- lib/numeric/modArith.cpp
+++ lib/numeric/modArith.cpp
@@ -9,29 +9,29 @@
 static const int MODVAL = 1000000007; // must be prime for op/
 struct mint
 {
 	int val;
 	mint():val(0){}
-	mint(int x):val(x%MODVAL) {}
-	mint(LL  x):val(x%MODVAL) {}
+	mint(int    x):val(x%MODVAL) {} // x>=0
+	mint(size_t x):val(x%MODVAL) {} // x>=0
+	mint(LL     x):val(x%MODVAL) {} // x>=0
 };
-mint operator+(mint x, mint y) { return x.val+y.val; }
-mint operator-(mint x, mint y) { return x.val-y.val+MODVAL; }
-mint operator*(mint x, mint y) { return LL(x.val)*y.val; }
-mint POW(mint x, int e) {
-	mint v = 1;
-	for(;e;x=x*x,e>>=1)
-		if(e&1)
-			v=v*x;
-	return v;
-}
-mint operator/(mint x, mint y) { return x * POW(y, MODVAL-2); }
+mint& operator+=(mint& x, mint y) { return x = x.val+y.val; }
+mint& operator-=(mint& x, mint y) { return x = x.val-y.val+MODVAL; }
+mint& operator*=(mint& x, mint y) { return x = LL(x.val)*y.val; }
+mint POW(mint x, LL e) { mint v=1; for(;e;x*=x,e>>=1) if(e&1) v*=x; return v; }
+mint& operator/=(mint& x, mint y) { return x *= POW(y, MODVAL-2); }
+mint operator+(mint x, mint y) { return x+=y; }
+mint operator-(mint x, mint y) { return x-=y; }
+mint operator*(mint x, mint y) { return x*=y; }
+mint operator/(mint x, mint y) { return x/=y; }
+vector<mint> FAC_(1,1);
+mint FAC(LL n) { while( FAC_.size()<=n ) FAC_.push_back( FAC_.back()*FAC_.size() ); return FAC_[n]; }
+mint C(LL n, LL k) { return k<0 || n<k ? 0 : FAC(n) / (FAC(k) * FAC(n-k)); }
+
 
-vector<mint> FAC_(1,1);
-void FAC_INIT(int n) { while( FAC_.size()<=n ) FAC_.push_back( FAC_.back()*FAC_.size() ); }
-mint FAC(mint x)       { return FAC_[x.val]; }
-mint C(mint n, mint k) { return k.val<0 || n.val<k.val ? 0 : FAC(n) / (FAC(k) * FAC(n-k)); }
+
 
 /*
 // MODVAL must be a prime!!
 LL GSS(LL k, LL b, LL e) // k^b + k^b+1 + ... + k^e
 {