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

subst(RAT[,VARN,RATN]*)
psubst(RAT[,VAR,RAT]*)
     :: RAT  VARN  RATN (N=1,2,... ōEɏ).

RETURN
     L
RAT,RATN
     L
VARN
     s茳

   * L̓̕s茳, 萔邢͑, LȂǂ
     ̂ɗp.
   * subst(RAT,VAR1,RAT1,VAR2,RAT2,...) ,
     subst(subst(RAT,VAR1,RAT1),VAR2,RAT2,...) ƓӖł.
   * ͂̍珇ɑJԂ߂, ͂̏ɂČʂ
     邱Ƃ.
   * `subst()' , `sin()' Ȃǂ̔̈ɑ΂Ăs. 
     `psubst()' , ̂悤Ȕ̓Ɨs茳ƌȂ, 
     ̈ɂ͍͑sȂ. (partial substitution ̂)
   * Asir ł, L̖񕪂͎Iɂ͍sȂ, L̑, 
     vʌvZԂ̑Nꍇ. Lꍇ
     ɂ, ɉƎ̔, Ȃׂ, q傫
     ȂȂ悤ɔz邱Ƃ΂ΕKvƂȂ.
   * ꍇlł.

     [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

