General Terms

License is granted to copy or use this document according to the Open Public License (OPL, enclosed below), a Public License, developed by the GNU Foundation.

Note well that there are three modifications to the usual OPL:

1. Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder, Robert G. Brown, who can be contacted for this or any other purpose related to this license at rgb at phy dot duke dot edu.

   (This is to prevent errors from being introduced which would reflect badly on the author's professional abilities.)

2. Distribution of this work or any derivative of the work in any for-profit form, e.g. paper or digital/electronic, is prohibited unless prior permission is obtained from the author/copyright holder.

   (This is so that the author can make at least some money if this work is republished as a paper or electronic book sold commercially for somebody else's profit. Note well that the author doesn't care about copies photocopied or locally printed and distributed free or at cost to students to support a course, reposted on the internet, chain mailed to friends, etc. except as insofar as the next modification is concerned.)

3. Any user of a free copy of this Open Publication Licensed material shall, upon meeting the author of this copyrighted material for the first time under the appropriate circumstances, offer to buy him or her or them a beverage. This beverage may or may not be alcoholic, depending on the personal ethical and moral views of the offerer(s) and receiver. The beverage cost need not exceed one U.S. dollar (although it certainly may at the whim of the offerer:-) and may be accepted or declined with no further obligation on the part of the offerer. It is not necessary to repeat the offer after the first meeting, but it can't hurt...