Implement a find() macro

This macro looks at each cell along some stride through the sheet,
  returning the location of the first cell encountered at which a given
  expression evaluates to true, or boolean false otherwise.

  To make this more useful, this change also revamps the handling of
  boolean values, making them much more permissive and coercing to boolean type
  in fewer circumstances. It adds the bool() type conversion for explicitly
  coercing to boolean type, and fixes a small issue with reading hex-formatted
  floating-point values ('p' can be used as an exponent indicator).

  Closes #64.

doc/teapot.lyx

@@ -4960,7 +4960,7 @@ and
 
 \series default
 \emph default
-y evaluates to the logical conjunction of boolean values 
+y evaluates to the logical conjunction of values 
 \emph on
 x
 \emph default
@@ -4979,6 +4979,32 @@ y
 \emph default
  is never evaluated, and so does not affect the value even if it is an error,
  for example.
+ In determining the value, the boolean conversions of 
+\emph on
+x
+\emph default
+ and 
+\emph on
+y
+\emph default
+ are used (see the description of the bool() function below), but the actual
+ value returned is either that of 
+\emph on
+x
+\emph default
+ or 
+\emph on
+y
+\emph default
+, namely, 
+\emph on
+x
+\emph default
+ if it corresponds to a boolean false value or 
+\emph on
+y
+\emph default
+ otherwise.
 \end_layout
 
 \begin_layout Description
@@ -5024,6 +5050,15 @@ and
 x
 \emph default
  is true.
+ Also in similar fashion, the value is 
+\emph on
+x
+\emph default
+ if it corresponds to a boolean true value and 
+\emph on
+y
+\emph default
+ otherwise.
 \end_layout
 
 \begin_layout Description
@@ -6442,24 +6477,41 @@ or
 \end_layout
 
 \begin_layout Description
-bool currently only acts as a keyword to 
+
+\series medium
+bool
+\begin_inset space ~
+\end_inset
+
+
+\series default
+bool
+\series medium
+[([
+\emph on
+x
+\emph default
+])]
+\series default
+ Converts 
+\emph on
+x 
+\emph default
+to a boolean value.
+ This is very permissive; error values are unaffected; empty, integer 0,
+ float 0.0, and boolean false values are false, and everything else is true.
+ If 
+\emph on
+x
+\emph default
+ is omitted, defaults to the value of the current cell.
+ If the parentheses are omitted as well, acts as a keyword, for example
+ for testing types with 
 \family sans
 \series bold
 is
 \series default
-(); 
-\family default
-there are not currently any conversions to boolean type.
- Use e.g.
-
-\family sans
- 
-\shape italic
-expr
-\shape default
- != 0 
-\family default
-to obtain a boolean value from a numerical expression.
+().
 \end_layout
 
 \begin_layout Description
@@ -6755,6 +6807,69 @@ function identifier,
  There is probably little practical call for this type.
 \end_layout
 
+\begin_layout Description
+find
+\series medium
+(
+\emph on
+expr
+\emph default
+,
+\emph on
+
+\begin_inset space ~
+\end_inset
+
+
+\emph default
+location
+\emph on
+
+\begin_inset space ~
+\end_inset
+
+stride
+\emph default
+[,location
+\emph on
+
+\begin_inset space ~
+\end_inset
+
+start
+\emph default
+]) examines cells in turn, returning the location of the first one at which
+ 
+\series default
+\emph on
+expr
+\emph default
+ evaluates to true, or boolean false if the boundary of the sheet is encountered.
+ The second argument 
+\emph on
+stride
+\emph default
+ must not have all zero components, and is added to the location being examined
+ at each iteration.
+ Often it is useful to use a direction constant like 
+\family sans
+\series bold
+up
+\family default
+\series default
+ for the stride.
+ The search begins at location 
+\emph on
+start
+\emph default
+, which defaults to the current location plus the 
+\emph on
+stride
+\emph default
+ if it is not specified.
+ 
+\end_layout
+
 \begin_layout Description
 
 \series medium