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 +247,24 @@ \subsection{Strings}\label{strings}
 \end{lstlisting}
 \end{nonnormative}
 
+\subsection{Units of Literal Constants}\label{units-literal-constants}
+
+Literal \lstinline!Real! and \lstinline!Integer! constants are implicitly assumed to have \lstinline!unit = "1"! if they appear in expressions containing variables, parameters, or constants that have a non-empty \lstinline!unit! attribute string.
+
+The inputs of transcendental 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 they involve expressions containing variables, parameters, or constants that have a non-empty unit attribute string.
+Their outputs are implicitly assumed to have unit \lstinline!"1"! if they are called within expressions containing variables, parameters, or constants that have a non-empty \lstinline!unit! attribute string.
+
+\begin{nonnormative}
+Rationale: unit attributes allow to check dimensional consistency of expressions and equations, as long as all the involved terms have a non-empty \lstinline!unit! attribute.
+Non-dimensional literal factors, e.g., \lstinline!2! in \lstinline!v = sqrt(2*g*h)! in physical laws involving quantities with units are always meant to be dimensionless, so they should be considered as such by giving them \lstinline!unit = "1"!.
+This allows to find out that, e.g., \lstinline!v = sqrt(2*h)! is dimensionally inconsistent (hence most likely wrong), because it has unit \lstinline!"m/s"! on the LHS and unit \lstinline!"m0.5"! on the RHS, which are not consistent.
+Without this rule, \lstinline!2! would have the default \lstinline!unit = ""! of \lstinline!Real!, making it impossible to determine the RHS unit.
+
+On the other hand, literal constants showing up in expressions containing only literal constants should be left with the default \lstinline!unit = ""! attribute, otherwise declarations such as \lstinline!Real L(unit = "m") = 2.0!, equations such as \lstinline!L = 2.0! and function calls such as \lstinline!temperature_ph(1e5, 293.15)! or \lstinline!temperature_ph(1e5, 273.15 + 20)! would be erroneously flagged as dimensionally inconsistent.
+
+The rules involving built-in transcendental 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