Newbie seeks help with evalution of forms

Hello all.
I am trying to write a function to calculate the error in the
approximation to the area under a curve. I am using the rectangle
approximation method. It turns out that the error is 
of the form a/n. a has a value which I have a function to calculate.
It needs a formula for the curve as an argument. 
This is the yform

/*
* The first thing to do is get the error in a one 
* rectangle approximation of the area under yform.
* yform: The formula for the curve
* lo: lower limit of area for approximation 
* hi: upper limit of area for approximation
* 
* returns the same type as rat.   
*/
toterr(yform,lo,hi) := block(
   [n:1],
   steps : makelist((lo + i*((hi-lo)/n)),i,0,n-1),
   errors : create_list(((hi - lo)/n)*
      (yform(j+((hi-lo)/n)) - yform(j)),
      j,steps),
   rat(apply("+",errors))
);

/* What I want to do below is, having got the number from the func 
 * above, i want to use the result in  an inequality.
 * -- Dons asbestos long-johns --
 * The trouble is, i think some part of the evaluation is broken,
 * and ev's docs are THE MOST CONFUSING THING I HAVE SEEN IN _N YEARS_
 * --End of flame-fest-- 
 */ 

find_n(yform,lo,hi, d,q) := block(
   [eq: yform],
   err : toterr(yform,lo,hi),
   err/n < d/q
);
(%o7) toterr(yform,lo,hi):=
(%o8) find_n(yform,lo,hi,d,q):=
block([eq:yform],err:toterr(yform,lo,hi),err/n<d/q)

(%i10) find_n(x^2,-2,2,1,100);
yform evaluates to x^2
Improper name or value in functional position.
#0: toterr(yform=x^2,lo=-2,hi=2)
#1: find_n(yform=x^2,lo=-2,hi=2,d=1,q=100)
 -- an error.  To debug this try debugmode(true);
(%i11) 

Please can someone correct my ignorance, and please forgive me.

Best Rob Grieve
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