=
"Specify an analytic solution to impose its expansion at the excision "
"surface. The analytic solution will be evaluated at coordinates "
"centered at the apparent horizon. "
"If the excision surface lies within the solution's "
"apparent horizon, the imposed expansion will be negative and thus the "
"excision surface will lie within an apparent horizon. Alternatively, "
"set this option to 'None' to impose the expansion is zero at the "
"excision surface, meaning the excision surface _is_ an apparent "
"horizon."