Add functions for unit displacements in the siz cardinal directions

This change is the "easy" part of #64, but to reduce the amount of duplicated
   code, this change also modifies the internals of implementing a function.
   The function identifier is now passed to the implementation as well, allowing
   many implementations of similar functions to be collapsed more easily.

doc/teapot.lyx

@@ -5900,7 +5900,65 @@ env
 \series default
 evaluates to the contents of the specified environment variable.
  If the variable does not exist, then an empty string will be returned.
+\end_layout
+
+\begin_layout Description
+above|below|left|right|up|down
+\series medium
+[([location
+\emph on
+
+\begin_inset space ~
+\end_inset
+
+l
+\emph default
+])] As bare words (without any arguments and also without the parentheses)
+ these symbols return a one-cell displacement in the named direction, e.g.
  
+\family sans
+\series bold
+D
+\series default
+(above) 
+\family default
+\series medium
+gives the location of the same cell as the current on but in the previous
+ layer and 
+\family sans
+\series bold
+R
+\series default
+(up)
+\family default
+\series medium
+ yields the value of the cell one up from the current cell.
+ With parentheses, they act like 
+\series bold
+@
+\series default
+()
+\series medium
+ but displaced by one cell in the named direction, so 
+\family sans
+\series default
+right()
+\family default
+\series medium
+ returns the value of the cell immediately to the right of the current one,
+ and 
+\family sans
+\series default
+down(MYTABLE)
+\family default
+\series medium
+ gives the value one line down from the cell labeled 
+\family sans
+\series default
+MYTABLE
+\family default
+\series medium
+, etc.
 \end_layout
 
 \begin_layout Description
@@ -6153,6 +6211,30 @@ x
  is given in radians.
 \end_layout
 
+\begin_layout Description
+below
+\series medium
+[([location
+\emph on
+
+\begin_inset space ~
+\end_inset
+
+l
+\emph default
+])] displacement by one cell in the positive 
+\series default
+\emph on
+z
+\emph default
+ direction; see the fuller description at 
+\family sans
+\series bold
+above
+\series default
+.
+\end_layout
+
 \begin_layout Description
 
 \series medium
@@ -6385,6 +6467,30 @@ x
  
 \end_layout
 
+\begin_layout Description
+down
+\series medium
+[([location
+\emph on
+
+\begin_inset space ~
+\end_inset
+
+l
+\emph default
+])] displacement by one cell in the positive 
+\series default
+\emph on
+y
+\emph default
+ direction; see the fuller description at 
+\family sans
+\series bold
+above
+\series default
+.
+\end_layout
+
 \begin_layout Description
 
 \series medium
@@ -6809,6 +6915,30 @@ trunc
  value is returned.
 \end_layout
 
+\begin_layout Description
+left
+\series medium
+[([location
+\emph on
+
+\begin_inset space ~
+\end_inset
+
+l
+\emph default
+])] displacement by one cell in the negative 
+\series default
+\emph on
+x
+\emph default
+ direction; see the fuller description at 
+\family sans
+\series bold
+above
+\series default
+.
+\end_layout
+
 \begin_layout Description
 
 \series medium
@@ -7248,6 +7378,30 @@ x
  
 \end_layout
 
+\begin_layout Description
+right
+\series medium
+[([location
+\emph on
+
+\begin_inset space ~
+\end_inset
+
+l
+\emph default
+])] displacement by one cell in the positive 
+\series default
+\emph on
+x
+\emph default
+ direction; see the fuller description at 
+\family sans
+\series bold
+above
+\series default
+.
+\end_layout
+
 \begin_layout Description
 
 \series medium
@@ -7882,6 +8036,23 @@ x
 ).
 \end_layout
 
+\begin_layout Description
+up
+\series medium
+[([location
+\emph on
+
+\begin_inset space ~
+\end_inset
+
+l
+\emph default
+])] displacement by one cell in the negative 
+\series default
+\emph on
+y
+\end_layout
+
 \begin_layout Description
 
 \series medium
@@ -8405,28 +8576,61 @@ func
  – in the former case it receives an argument count of 0, and in the latter
  an argument count of -1.
  So the same symbol can have different semantics depending on which way
- it is called, but in most cases that would probably be counterintuitive.
- The no-parentheses form is useful, however, for constants like 
+ it is called.
+ In most cases such a difference is counterintuitive, but it seems to work
+ fairly well for the directional 
 \begin_inset Quotes eld
 \end_inset
 
-e,
+constants,
 \begin_inset Quotes erd
 \end_inset
 
- and it is also used to allow, for example, the names of the functions 
+ i.e.
+ bare 
 \begin_inset Quotes eld
 \end_inset
 
-floor, 
-\begin_inset Quotes eld
-\end_inset
-
-round,
+left
 \begin_inset Quotes erd
 \end_inset
 
- etc.
+ is the displacement &(-1,0,0), whereas left() is the location one to the
+ left.
+ The no-parentheses form is also useful for constants like 
+\begin_inset Quotes eld
+\end_inset
+
+e
+\begin_inset Quotes erd
+\end_inset
+
+ and 
+\begin_inset Quotes eld
+\end_inset
+
+tau
+\begin_inset Quotes erd
+\end_inset
+
+, and it is also used, for example, to allow the names of the functions
+ 
+\begin_inset Quotes eld
+\end_inset
+
+floor
+\begin_inset Quotes erd
+\end_inset
+
+, 
+\begin_inset Quotes eld
+\end_inset
+
+round
+\begin_inset Quotes erd
+\end_inset
+
+, etc.
  to be used as keywords for the int() function.
 \end_layout