Indication of which end of
the string should be
trimmed or extended to the
bounding object, this
should be set to one of
the following values:

UF_CURVE_TRIM_EXTEND_START
UF_CURVE_TRIM_EXTEND_END

A character string containing the value to
set the total length of the input string to.
The string can be a numeric value (e.g.,
"25"), a fully defined expression (e.g.,
"p5 = 25"), an existing expression name
(e.g., "p5"), or an expression string (e.g.,
"p5+p7"). If the fully defined expression
exists then an error is returned. If the
expression name does not exist an error is
returned. In all other cases an expression
is created. When this value is returned
from UF_CURVE_ask_trim, the character
string is a fully defined expression
(e.g., "p5=25").