/* rpn udf:  trapiz
/* purpose:  trapizoid rule integration
/* user's function is "fn"
/* lower limit stored in "a"
/* upper limit stored in "b"
/* number of panels stored in "n"
/* call to do integral is "trapiz"
/*
/* Michael Borland, 1988
0 sto a sto b sto dx sto n sto i
  sto sum sto ilim
udf
trapiz
b a - n / sto dx 
a + sto x  
1 sto i  
n 1 - sto ilim 
0 sto sum 
cle 
sum_terms

udf
sum_terms
i ilim == cle ? fini : x fn sum + sto sum x dx + sto x i 1 + sto i sum_terms $

udf
fini
a fn b fn + 2 / sum + dx * sto sum cle sum