`str_len', `str_chr', `sub_str'
-------------------------------

str_len(STR)
     :: $BJ8;zNs$ND9$5$rJV$9(B.

str_chr(STR,START,C)
     :: $BJ8;z$,:G=i$K8=$l$k0LCV$rJV$9(B.

sub_str(STR,START,END)
     :: $BItJ,J8;zNs$rJV$9(B.

RETURN
     `str_len()', `str_chr()':$B@0?t(B; `sub_str()':$BJ8;zNs(B

STR,C
     $BJ8;zNs(B

START,END
     $BHsIi@0?t(B

   * `str_len()' $B$OJ8;zNs$ND9$5$rJV$9(B.

   * `str_chr()' $B$O(B STR $B$N(B START $BHVL\$NJ8;z$+$i%9%-%c%s$7$F(B $B:G=i$K(B C
     $B$N:G=i$NJ8;z$,8=$l$?0LCV$rJV$9(B. $BJ8;zNs$N@hF,$O(B 0 $BHVL\$H$9$k(B.
     $B;XDj$5$l$?J8;z$,8=$l$J$$>l9g$K$O(B -1 $B$rJV$9(B.

   * `sub_str()' $B$O(B, STR $B$N(B START $BHVL\$+$i(B END $BHVL\(B
     $B$^$G$NItJ,J8;zNs$r@8@.$7JV$9(B.

     [185] Line="123 456 (x+y)^3";
     123 456 (x+y)^3
     [186] Sp1 = str_chr(Line,0," ");
     3
     [187] D0 = eval_str(sub_str(Line,0,Sp1-1));
     123
     [188] Sp2 = str_chr(Line,Sp1+1," ");
     7
     [189] D1 = eval_str(sub_str(Line,Sp1+1,Sp2-1));
     456
     [190] C = eval_str(sub_str(Line,Sp2+1,str_len(Line)-1));
     x^3+3*y*x^2+3*y^2*x+y^3

