Relativity theory in Lojban
According to lai krtisfranks, this is how Lojban currently does it (for a given inertial frame of reference). Note that any line parallel to the x' line (such as the gold one and, notably/particularly, the x' line itself) is a line of simultaneity (ca). Any event in the region below the x' line (the region with the tight, purple, vertical stripes) is in the "past"/pu of the observer whose reference frame we are examining; any event in the region above the x' line (the region with the wide, blue, horizontal stripes) is in the "future"/ba of the observer. It is important to understand that all pastward timelike-separated (and lightlike-separated) events are in the "past" (for everyone, no matter what reference frame), but some spacelike-separated evens are also in this observer's particular "past" region; likewise for their "future" region; the events that are spacelike-separated can be apparently reordered so that they are simultaneous or even have their apparent order switched in some frames of reference (but nothing within the light-cone has this property, with strictness matching the strictness of what "within" means). This picture varies for each frame of reference. But it is representative of all of them in the broad strokes.
The maroon discs each are events (they are pointlike, but I made them seem larger- also, the variations in the size of the discs is just my fault and means nothing). The gold lines are signal. The vertical grey and black lines are observers who happen to share a frame of reference. The blue lines are the division between "past" and "future" according to non-Relativistic interpretation.
Let the signal channel be snailmail, event A be the event of Grey sending notice that he got attacked by a bear, event b be the event of Black receiving the letter and sending back (instantaneously in this example) his condolences, and event C be the event of Grey receiving the communication of said condolences.
[Please check everything that I am saying. I noticed halfway through that I needed to rewrite things, so I may have missed some words/meanings or messed up some of the grammar by copy-paste.] le mu'e la .grusi. bejgau lo xatra be fo le nu lo cribe la .grusi. gunta kei be'o goi xy xi pa la .xekri kei .goi tau .a'y cabna le mu'e la .xekri. co'e xi pa kei goi .a'y ma'i tu'a la .grusi. joi la .xekri. goi my
.i le mu'e la .grusi. co'e xi re kei goi tau by cabna le mu'e la .xekri. terbejgau xy xi pa gi'e bejgau lo xatra be fo le nu la
.xekri. kecti la .grusi. be'o goi xy xi re kei goi by ma'i my
.i le mu'e la .grusi. terbejgau xy xi re kei goi tau cy cabna le mu'e le .xekri. co'e xi ci kei goi cy ma'i my
.i by .e cy .e tau by .e tau cy balvi .a'y .e tau .a'y ma'i my .i .a'y .e tau .a'y purci by .e cy .e tau by .e tau cy ma'i my
.i cy .e tau cy balvi by .e tau by ma'ia my .i by .e tau by purci cy .e tau cy ma'i my
.i .a'y .e tau .a'y purci by .e tau by ma'i my .i by .e tau by balvi .a'y .e tau .a'y ma'i my
.i tau .a'y boi by tcese di'au tu'a lo masno se'o'e mrilu co'e goi zy
.i by boi tau cy tcese .i to .a'y tcese tau by .i tau by tcese cy toi
.i tau .a'y calse .a'y di'au zy .i tau by calse by .i tau cy calse cy
.i tau .a'y purci temse cy .e tau by .e tau cy di'au zy .i cy .e tau by .e tau cy balvi temse tau .a'y
.i .a'y purci temse tau cy .e by .e cy .i tau cy .e by .e cy balvi temse .a'y
.i tau by purci temse tau cy .i tau cy balvi temse tau by
.i by purci temse cy .i cy balvi temse by