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

subst(RAT[,VARN,RATN]*)
psubst(RAT[,VAR,RAT]*)
     :: RAT  VARN  RATN (N=1,2,... Ǻ鱦˽缡).

RETURN
     ͭ
RAT,RATN
     ͭ
VARN
     긵

   * ͭ긵, 뤤¿༰, ͭʤɤ
     ΤѤ.
   * subst(RAT,VAR1,RAT1,VAR2,RAT2,...) ,
     subst(subst(RAT,VAR1,RAT1),VAR2,RAT2,...) Ʊ̣Ǥ.
   * Ϥκ¦򷫤֤, ϤνˤäƷ̤
     뤳Ȥ.
   * `subst()' , `sin()' ʤɤȡΰФƤԤ. 
     `psubst()' , Τ褦ȡĤΩ긵ȸʤ, 
     ΰˤϹԤʤ. (partial substitution ΤĤ)
   * Asir Ǥ, ͭʬϼưŪˤϹԤʤ, ͭ, 
     פ̷׻֤礬. ͭ
     ˤ, ˱ȼȡ񤤤, ʤ٤ʬ, ʬҤ礭
     ʤʤ褦θ뤳Ȥ⤷ФɬפȤʤ.
   * ʬƱͤǤ.

     [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

