These modules were initially created by IPGG in Paris in the 1990s and eventually donated to us for continued upkeep. Because no guidelines for option selection existed at the time, I have had to make major changes over the years, such as preventing that these modules eat up available common options for the rest of GMT (e.g., -o, etc). Yet, we still have some discordant notes in the system. Today we fixed coupe -Zcpt to be -Ccpt like in the rest of GMT. However, meca also uses -Z that way, and -C is used and does something else. I have looked at the four modules, determined available option codes, and looked for simplicities. Here is my assessment and proposal for an update. The update will be backwards compatible, of course.

1. CPT color are optionally needed by meca, coupe, might be a very useful addition for velo, and possibly for polar if there is a use for this. Thus, I wish to make space for -C for all four, wether they presently use a CPT or not.
2. meca and polar have a similar -C option, but with some differences: In meca, -C forces us to read two extra columns with an alternative lon,lat location where the symbol will be plotted, and then a line is drawn from that location to the original location. Optionally, a small circle (pen only) is drawn at the original location. For polar, there really is no lon,lat input so -D is used to specify where on the map you want this hemisphere plot to be centered. Its -C option then takes a second location and then the same thing happens: hemisphere plot is placed at -Clon/lat and a line with optional circle goes to the -Dlon/lat point. Here, I think it makes most sense to let the functionality of -C be just modifiers to -D instead, thus leaving -C free. For meca I think it is best not to clutter -S with more modifiers but simply introduce -A as the new option to take over for the old -C. That way can use +a with polar -D for some mnemonic value.
3. I guess because of the projection onto the crossection, coupe does not have a similar -C facility to adjust location of symbol. Might this be conceivable useful to allow for, @seisman ? If it is, then uniformity across these modules would imply we use the common -Z letter (and +z in polar) instead of -A.
4. Of course, the other issue with the existing -C option is that one takes a pen while the other takes a +ppen modifier. Both take an optional +ssize to affect the circle size.

Proposal:

1. Make all these module have -Ccpt or have the capacity to add one
2. Depending on the coupe question, let ? be either A or Z. Then convert meca -C to meca -? and polar -C to -D+?.
3. Use +ppen modifier in both cases, but still honor pen when in backwards mode.

Please comment, @GenericMappingTools/core.