`subst', `psubst'
-----------------

subst(RAT[,VARN,RATN]*)

psubst(RAT[,VAR,RAT]*)
     :: RAT $B$N(B VARN $B$K(B RATN $B$rBeF~(B (N=1,2,... $B$G:8$+$i1&$K=g<!BeF~$9$k(B).

RETURN
     $BM-M}<0(B

RAT,RATN
     $BM-M}<0(B

VARN
     $BITDj85(B

   * $BM-M}<0$NFCDj$NITDj85$K(B, $BDj?t$"$k$$$OB?9`<0(B,
     $BM-M}<0$J$I$rBeF~$9$k$N$KMQ$$$k(B.

   * subst(RAT,VAR1,RAT1,VAR2,RAT2,...) $B$O(B,
     subst(subst(RAT,VAR1,RAT1),VAR2,RAT2,...)  $B$HF1$80UL#$G$"$k(B.

   * $BF~NO$N:8B&$+$i=g$KBeF~$r7+$jJV$9$?$a$K(B,
     $BF~NO$N=g$K$h$C$F7k2L$,JQ$o$k$3$H$,$"$k(B.

   * `subst()' $B$O(B, `sin()' $B$J$I$NH!?t$N0z?t$KBP$7$F$bBeF~$r9T$&(B.
     `psubst()' $B$O(B, $B$3$N$h$&$JH!?t$r0l$D$NFHN)$7$?ITDj85$H8+$J$7$F(B, $B$=(B
     $B$N0z?t$K$OBeF~$O9T$o$J$$(B. (partial substitution $B$N$D$b$j(B)

   * Asir $B$G$O(B, $BM-M}<0$NLsJ,$O<+F0E*$K$O9T$o$J$$$?$a(B, $BM-M}<0$NBeF~$O(B,
     $B;W$o$L7W;;;~4V$NA}Bg$r0z$-5/$3$9>l9g$,$"$k(B.
     $BM-M}<0$rBeF~$9$k>l9g$K$O(B, $BLdBj$K1~$8$?FH<+$NH!?t$r=q$$$F(B,
     $B$J$k$Y$/J,Jl(B,
     $BJ,;R$,Bg$-$/$J$i$J$$$h$&$KG[N8$9$k$3$H$b$7$P$7$PI,MW$H$J$k(B.

   * $BJ,?t$rBeF~$9$k>l9g$bF1MM$G$"$k(B.

     [0] subst(x^3-3*y*x^2+3*y^2*x-y^3,y,2);
     x^3-6*x^2+12*x-8
     [1] subst(@@,x,-1);
     -27
     [2] subst(x^3-3*y*x^2+3*y^2*x-y^3,y,2,x,-1);
     -27
     [3] subst(x*y^3,x,y,y,x);
     x^4
     [4] subst(x*y^3,y,x,x,y);
     y^4
     [5] subst(x*y^3,x,t,y,x,t,y);
     y*x^3
     [6] subst(x*sin(x),x,t);
     sint(t)*t
     [7] psubst(x*sin(x),x,t);
     sin(x)*t

