\n\
A field is a region of text with the same `field' property.\n\
If NEW-POS is nil, then the current point is used instead, and set to the\n\
-constrained position if that is is different.\n\
+constrained position if that is different.\n\
\n\
If OLD-POS is at the boundary of two fields, then the allowable\n\
positions for NEW-POS depends on the value of the optional argument\n\