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

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

RETURN
     $@M-M}<0(J
RAT,RATN
     $@M-M}<0(J
VARN
     $@ITDj85(J

   * $@M-M}<0$NFCDj$NITDj85$K(J, $@Dj?t$"$k$$$OB?9`<0(J, $@M-M}<0$J$I$rBeF~$9$k(J
     $@$N$KMQ$$$k(J.
   * subst(RAT,VAR1,RAT1,VAR2,RAT2,...) $@$O(J,
     subst(subst(RAT,VAR1,RAT1),VAR2,RAT2,...) $@$HF1$80UL#$G$"$k(J.
   * $@F~NO$N:8B&$+$i=g$KBeF~$r7+$jJV$9$?$a$K(J, $@F~NO$N=g$K$h$C$F7k2L$,JQ(J
     $@$o$k$3$H$,$"$k(J.
   * `subst()' $@$O(J, `sin()' $@$J$I$NH!?t$N0z?t$KBP$7$F$bBeF~$r9T$&(J. 
     `psubst()' $@$O(J, $@$3$N$h$&$JH!?t$r0l$D$NFHN)$7$?ITDj85$H8+$J$7$F(J, $@$=(J
     $@$N0z?t$K$OBeF~$O9T$o$J$$(J. (partial substitution $@$N$D$b$j(J)
   * Asir $@$G$O(J, $@M-M}<0$NLsJ,$O<+F0E*$K$O9T$o$J$$$?$a(J, $@M-M}<0$NBeF~$O(J, 
     $@;W$o$L7W;;;~4V$NA}Bg$r0z$-5/$3$9>l9g$,$"$k(J. $@M-M}<0$rBeF~$9$k>l9g(J
     $@$K$O(J, $@LdBj$K1~$8$?FH<+$NH!?t$r=q$$$F(J, $@$J$k$Y$/J,Jl(J, $@J,;R$,Bg$-$/(J
     $@$J$i$J$$$h$&$KG[N8$9$k$3$H$b$7$P$7$PI,MW$H$J$k(J.
   * $@J,?t$rBeF~$9$k>l9g$bF1MM$G$"$k(J.

     [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

