Added "four-line" proposal for units of literals

RationaleMCP/0027/ReadMe.md

@@ -0,0 +1,34 @@
+# Modelica Change Proposal MCP-0027<br/>Units of Literal Constants
+Francesco Casella, Henrik Tidefelt
+
+**(In Development)**
+
+## Summary
+The purpose of this MCP is to allow more unit errors to be detected by giving more expressions the unit `"1"` instead of having an undefined unit.
+The problem with undefined unit is that it gets in the way of carrying out checking of units (which tools tend to deal with by not checking units at all where this happens).
+
+## Revisions
+| Date | Description |
+| --- | --- |
+| 2022-10-04 | Henrik Tidefelt. Filling this document with initial content. |
+
+## Contributor License Agreement
+All authors of this MCP or their organizations have signed the "Modelica Contributor License Agreement".
+
+## Rationale
+FIXME
+
+## Backwards Compatibility
+As current Modelica doesn't clearly reject some models with non-sensical combination of units, this MCP will break backwards compatibility by turning at least some of these invalid.
+
+## Tool Implementation
+None, so far.
+
+### Experience with Prototype
+N/A
+
+## Required Patents
+To the best of our knowledge, there are no patents that would conflict with the incorporation of this MCP.
+
+## References
+(None.)

chapters/lexicalstructure.tex

@@ -247,6 +247,29 @@ \subsection{Strings}\label{strings}
 \end{lstlisting}
 \end{nonnormative}
 
+\subsection{Units of Literal Constants}\label{units-literal-constants}
+
+The following rules apply:
+\begin{itemize}
+\item The result of \lstinline!x + L!, \lstinline!L + x\lstinline!, \lstinline!x - L!, \lstinline!L - x!, \lstinline!x*L!, \lstinline!L*x!, \lstinline!x/L!, where \lstinline!x! is an expression with non-empty \lstinline!unit! attribute string `<s>` and \lstinline!L! is an expression containing only literal constants, shall have the same \lstinline!unit! attribute string of \lstinline!x!.
+\item The result of \lstinline!L/x\lstinline! shall have the \lstinline!unit! attribute string `1/(<s>)`.
+\item The result of \lstinline!x^L! shall have the \lstinline!unit! attribute string of \lstinline!product(fill(x, L))! if L is a positive Integer, or the unit of \lstinline!1/product(fill(x, L))! if L is a negative Integer.
+\item If either side of a relational operator is a literal constant, then it is assumed to have the same \lstinline!unit! attribute string of the other side, if that is well-defined.
+\end{itemize}
+
+The inputs of the elementary mathematical functions defined in \cref{built-in-mathematical-functions-and-external-built-in-functions} such as \lstinline!sin()!, \lstinline!cos()!, \lstinline!exp()!, etc., should have a dimensionless unit (e.g., \lstinline!"1"! or \lstinline!"rad"!) if the unit string of the input is non-empty. Function \lstinline!atan2(y,x)! should be treated as  \lstinline!atan(x/y)! for dimensional consistency checking purposes.
+In that case, their outputs are also implicitly assumed to have a dimensionless unit.
+
+\begin{nonnormative}
+Rationale: by default, literal Real and Integer constants do not have a defined, non-empty \lstinline!unit! string attribute; hence they act as "unit wildcards", preventing dimensional consistency checking of equations that contain them.
+The rules regarding multiplication and division prevent this effect, allowing, e.g., to determine that \lstinline!v = sqrt(2*g*h)! is dimensionally consistent, while \lstinline!v = sqrt(2*g)! is dimensionally inconsistent, and thus most likely wrong.
+The rules regarding addition and subtraction instead allow to perform some basic unit inference in expressions containing mixed literal constants and variables (when there are no ambiguities in doing so), again expanding the scope for dimensional consistency checking.
+For example, they allow to determine that \lstinline!tau = L/(abs(v) + 1e-9)! is dimensionally consistent, while \lstinline!tau = 1/(abs(v) + 1e-9)! is not.
+
+
+The rules involving elementary mathematical functions extend the scope of this concept to also allow determining that equations such as \lstinline!i = i0*exp(v/i0)! or \lstinline!i = v0*exp(i/i0)! are dimensionally inconsistent.
+\end{nonnormative}
+
 \section{Operator Symbols}\label{operator-symbols}
 
 The predefined operator symbols are formally defined on page \pageref{lexical-conventions} and