TSUM

       Trapezoidal summation of the area under a curve.

       Adapted from the procedure INTEG in the IUE procedure library.

       Result = TSUM(y)
              or
       Result = TSUM( x, y, [ imin, imax ] )

       x = array containing monotonic independent variable.  If omitted, then
               x is assumed to contain the index of the y variable.
               x = lindgen( N_elements(y) ).
       y = array containing dependent variable y = f(x)

       imin = scalar index of x array at which to begin the integration
               If omitted, then summation starts at x[0].
       imax = scalar index of x value at which to end the integration
               If omitted then the integration ends at x[npts-1].

       result = area under the curve y=f(x) between x[imin] and x[imax].

       IDL> x = [0.0,0.1,0.14,0.3]
       IDL> y = sin(x)
       IDL> print,tsum(x,y)    ===>  0.0445843
       In this example, the exact curve can be computed analytically as
       1.0 - cos(0.3) = 0.0446635

       The area is determined of individual trapezoids defined by x[i],
       x[i+1], y[i] and y[i+1].
       If the data is known to be at all smooth, then a more accurate
       integration can be found by interpolation prior to the trapezoidal
       sums, for example, by the standard IDL User Library int_tabulated.pro.

       Written, W.B. Landsman, STI Corp. May 1986
       Modified so X is not altered in a one parameter call Jan 1990
       Converted to IDL V5.0   W. Landsman   September 1997
       Allow non-integer values of imin and imax  W. Landsman April 2001
       Fix problem if only 1 parameter supplied W. Landsman June 2002

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