in­vert­ed­para­graphs – In­dented text with neg­a­tive para­graph in­den­ta­tion

De­fines an en­vi­ron­ment that be­haves like a list, ex­cept that in­stead of an item la­bel, the para­graph starts at the left mar­gin, but car­ries on in­dented.
The macros (gra­tu­itously) load may­be­load and re­de­fine.