You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: forward_substitution.m
    path of file: ./repos/numerical-methods-matlab/linear_systems
    
    the code of the file until where you have to start completion: function [x] = forward_substitution(l, c)
    % Solve the lower linear system lx=c.
    %
    % Args:
    %     lower: lower triangular matrix.
    %     c: an array containing c values.
    %
    % Returns:

    
function [x] = forward_substitution(l, c)
    % Solve the lower linear system lx=c.
    %
    % Args:
    %     lower: lower triangular matrix.
    %     c: an array containing c values.
    %
    % Returns:
    %     x: solution of linear the system.

    [n, m] = size(l);

    if n ~= m
        error('Error: "l" must be a square matrix.')
    end